Chapter XIV
FOSSIL ICHTHYOLOGY; COMPRISING THE SHARKS, RAYS AND OTHER PLACOID FISHES.
Ascending from the two grand subdivisions of the animal kingdom, the Mollusca and the Articulata, we advance to the Vertebrata, animals distinguished from all those which have previously engaged our attention, by the possession of a bony, jointed, hollow column of support, or spine, formed of bones termed vertebræ (turn or whirl bones), and enclosing and protecting those strands or cords of the nervous system called the spinal marrow; the former classes, being destitute of such a structure, have the general name of Invertebrata.
In the beings whose mineralized remains form the subject of our present investigation, the durable parts of the frame-work, or skeleton, are, in most instances, situated internally, and their fossil relics consist principally of the bones, or solid earthy portions of their structures, either imbedded in the rocks in their natural relative position, or in a state of dismemberment and dispersion. In most cases the teeth, and in many the durable parts of their external integument, or skin, are also preserved, in a greater or less degree of integrity.
In the lowest class of vertebrata, the Fishes, the skin is covered with numerous pieces or scales, of a dense, durable substance, and strengthened, in some families, by the addition of osseous plates; thus constituting a flexible and almost impenetrable coat of armour, which affords suitable protection to beings peculiarly exposed to external injuries, from the nature of the regions they inhabit, and the state of warfare with each other in which they are constantly engaged. Confined to a fluid medium, they are provided with organs fitted for aquatic respiration, called branchiæ, or gills, and with instruments of progressive motion, termed fins, by which they are enabled to propel themselves through the water with great velocity. The apparatus for seizing, tearing, and crushing their prey presents numerous and important modifications, corresponding to the habits and economy of the different genera; their teeth offering as great variety of form and structure as those of the higher orders of animals.
The cartilaginous or the osseous nature of the skeleton, and the number and position of the fins, were the characters formerly employed in the classification of Fishes; but Prof. Agassiz, conceiving the structure of the skin to afford a natural index to the essential modifications of organization and functions, has, with great sagacity, adopted an arrangement founded upon the form and structure of the scales; and he has divided the whole class into four orders, each distinguished by essential differences in the dermal (skin) system. To the geologist this method has proved of inestimable value; for it is simple, easy of application, and, so far as our present knowledge extends, may be relied upon as affording accurate conclusions as to the nature and relations of the originals to which a few detached fossil scales may have belonged. Another important aid has been derived from the microscopical examination of the structure of the teeth; and a splendid work on this subject by Professor Owen has opened a wide field of palæontological investigation, which is yet but very partially explored.[505]
[505] Odontography; or, a Treatise on the Comparative Anatomy of the Teeth; their Physiological Relations, Mode of Development, and Microscopic Structure; illustrated by upwards of 150 plates. By Prof. R. Owen, F.R.S. &c. 4to. London. 1840-1845.
The living species of Fishes exceed eight thousand; and those found in a fossil state, and determined by M. Agassiz, already amount to upwards of one thousand five hundred; while several hundreds are still undescribed; and the rapid progress of geological research is continually adding to the number: upwards of six hundred British fossil species are enumerated. In an initiatory work like the present, it will be necessary to confine our remarks to an illustration of the mode in which the investigation of the fossil remains of the animals of this class should be conducted; and, by the elucidation of a few leading principles, prepare the student for the perusal of works expressly devoted to this branch of Palæontology.[506]
[506] The admirable and important work entitled "Recherches sur les Poissons Fossiles, par Louis Agassiz," stands preeminent in this department of science. It consists of five volumes, 4to. of letter-press, and five volumes, folio, of coloured plates. It must be consulted by all who would acquire a correct view of the present state of fossil Ichthyology. It is from this work that the commentary in the text has been chiefly derived.
The fossil remains of fishes rank in the first class of the "Medals of Creation," for they demonstrate the existence of numerous tribes of highly organized beings in some of the most ancient fossiliferous strata, and the continuance of the same type of organization, variously modified, through the entire series of subsequent deposits to the present time. Each geological formation contains peculiar groups of fossil fishes, distinguished by distinct modifications of structure. Thus, according to the data at present obtained, all the osseous fishes anterior to the Chalk belong to genera which have no representatives among existing species; and they are characterised by rhomboidal scales covered with enamel.
The state of conservation in which the fossils of this class occur, appears to have depended on the relative delicacy or firmness of the original structures, and on the nature of the deposits in which the fishes were imbedded. Thus the fossil fishes of the early formations, which are characterised by their dense integument and enamelled scales, often present the entire forms of the originals, and generally considerable portions of the connected scales, with the fins and other appendages: while the specimens of later deposits, which contain a large proportion of species with delicate scales, more often display the mineralized osseous skeleton, than the dermal structure. Sedimentary strata composed of mud or fine detritus, of whatever age, have been most favourable to the preservation of the entire forms; hence we often find in the pulverulent clays and marls of the Tertiary strata, in the Chalk of England and Westphalia, and in the fine lithographic stone of Solenhofen, fishes perfect in form, and not only individuals, but groups, with the scales, fins, head, teeth, and even the capsule of the eye, in their natural positions. A small slab of marl from Aix, in Provence, in the collection of Sir R. I. Murchison, contains scores of small fishes, as perfect as if recently imbedded in soft mud: a portion of this specimen is represented, Lign. 184; and the beautiful fish figured in the frontispiece of vol. i. (pl. i. fig. 3), from near Castellamare, will serve to illustrate the state of perfection of some of the ichthyolites of the Jura limestone. In the Chalk, many of the fishes are uncompressed, the body being as perfect in form as if the original had been surrounded by soft plaster of Paris while floating in the water. But in coarse limestones and conglomerates,--in other words, in materials that have been subjected to the action of the waves and torrents,--detached teeth, scales, bones, &c. constitute the principal vestiges of this class of beings.
In illustration of this department of Paleontology, it will be expedient to consider,--1stly, the characters afforded by the scales and dermal appendages; 2dly, the teeth, or dental organs; 3dly, the osseous and cartilaginous skeletons; and lastly, apply the data thus obtained to the elucidation of some of the principal fossil genera and species.
Scales of Fishes.--The dermal plates or scales are composed of two substances, disposed in laminæ or plates; the one cartilaginous or horny,--the other dense and osseous, possessing the structure of bone. In most species the scales are imbricated, i. e. lie over each other like the tiles of a roof; the margin of a front row partly covering the series immediately behind. From this arrangement, the apparent shape of the scales is very different from their true form; the processes of attachment and the lateral angles being concealed. The scales that are not imbricated are either very small, and imbedded in the substance of the skin so as to be imperceptible to the naked eye, as in the shagreen of Sharks; or are disposed in the form of bosses or scutcheons, as in the Rays; sometimes bristling equally over the surface of the body, as in the Diodon; and sometimes covering it like mosaic work; or forming particular series on certain regions of the body, while the other parts are garnished with different scales, as in the Sturgeon. There are a few genera destitute of scales. In almost all fishes there is a particular series disposed in a gently undulated line along each side of the body, from the head to the tail, and constituting what is termed the lateral line; these scales are tubular, and serve an important purpose in the economy of these animals. Every one must be aware that the body in most living fishes is constantly covered with a kind of mucus, or slime, which serves to lubricate the skin and to defend it from the action of the surrounding medium. This fluid is secreted by a mucous canal or duct, which extends along the body, and ramifies in all the bones of the head, jaws, &c.; and it is distributed over the surface of the head by numerous pores in the bones, and over the body by the tubes formed by the row of scales above described.
Fig. 1.--Scale or plate of the shagreen of a Shark. 2.--Scale of Macropoma Mantelli; the exposed surface. 3.--Scale of Beryx Lewesiensis; the exposed surface. 4.--Scale of Osmeroides Mantelli; the entire form.
The four orders into which this grand class of vertebrata is divided by M. Agassiz, are founded upon the peculiar structure of the scales;[507] and are characterised as follow:--
[507] For illustrations of the scales of fishes, see Wond. p. 339, Lign. 68, and p. 340, Lign. 69; Foss. Brit. Mus. p. 419; and Ly. fig. 306, figs. 342-347, &c. And for their minute structure, see Prof. Williamson’s important paper in the Phil. Trans. for 1851.
Order I. Placoid (a broad plate).--The skin covered irregularly with enamelled plates, sometimes of a large size, but frequently in small points, as the shagreen on the dermal integument of the Sharks and the tubercles of the Rays. Lign. 185, fig. 1, a fossil placoidian scale from the skin of a shark, highly magnified.
Order II. Ganoid (splendid, from the brilliant surface of the enamel).--The scales are of an angular form, and composed of plates of horn or bone, covered with a thick layer of enamel; their structure is identical with that of the teeth. The Sturgeon is an example of this order. Lign. 196, figs. 1, 2, 3, 4, are fossil scales of a ganoidian fish.
Order III. Ctenoid (toothed, or comb-like).--The scales are formed of plates, which are toothed or pectinated on their posterior margin or edge, like a comb. As the plates are superimposed on each other, so that the lowermost always extend beyond the uppermost, their numerous sharp points or teeth render the scales very harsh to the touch. The Perch belongs to this order. Lign. 185, fig. 3, represents a fossil ctenoidian scale.
Order IV. Cycloid (circular).--The scales are composed of simple laminæ, or plates of bone or horn, without enamel, and have smooth borders; but their external surface is often ornamented with markings. The scales of the lateral line consist of funnels placed one within the other; the contracted part of which, applied against the disk of the scale, forms the tube through which the mucus flows. To this order belong the Mullet, Salmon, and Carp. Lign. 185, fig. 4, is the scale of a fossil cycloidian fish.
Fins of Fishes.--As the progression of fishes through the water is principally effected by the action of the tail, they have no limbs commonly so called. The instruments for balancing the body, and for assisting progression, are the fins, which are composed of numerous rays that support a membranous expansion; and the number and situation of the fins present various modifications in the different orders and genera.
The fins are named according to the situation they occupy; for example, pectoral, those on each side of the chest, and which correspond to the anterior extremities of other vertebrated animals; dorsal, on the back; ventral, on the belly; caudal, on the tail. (See outlines of Fish, Ligns. 186, 187, 195.) The rays are of two kinds; 1st, the Spinous rays; these consist of a single osseous piece, usually dense and pointed, sometimes flexible and elastic, and divided longitudinally (Ligns. 188, 196); 2d, Soft or articulated rays, which are composed of numerous small articulations or joints, and divide into branches at their extremities. Many species of fishes have four fins; others six; some but two; and in certain genera they are altogether wanting. In a fossil state the fins are often beautifully preserved; even the soft rays in many of the Tertiary marls and in the Chalk, are found entire, and attached to the body in their natural situation. The large, strong, spinous rays of the dorsal fins of the cartilaginous fishes, as the Sharks and Rays, are generally found detached, or connected only with a few vertebræ; but they are so abundant in some of the Secondary deposits (and in numerous instances they are the only vestiges of extinct species and genera), that they possess great geological interest; they are distinguished by the term Ichthyodorulites (fossil fish-weapons), under which head they will hereafter be described (see Lign. 188). The first ray in the dorsal fin of some fishes is protected in front by a double row of enamelled scales, and these often occur in a fossil state (see Lign. 196, fig. 5).
Teeth of Fishes.--Of all the durable parts of animals teeth occur in the mineral kingdom, the teeth of fishes present by far the most numerous, varied, and striking modifications of form, structure, composition, mode of arrangement, and attachment; and yet these dental organs, separately considered, do not in many instances, either in their structure or mode of implantation, afford characters by which the natural affinities of the original can be satisfactorily ascertained; and without the aid of other parts of the skeleton it is often impossible to determine, from external characters only, whether an unknown form of tooth belonged to an animal of the class of Fishes or of Reptiles. Although the modifications of form are almost innumerable, they are referable to four principal types; namely, the conical, the flattened, the prismatic, and the cylindrical.[508]
[508] The "Odontography" of Professor Owen should be consulted by those who would thoroughly comprehend this interesting department of science. See also the Article Teeth, by Prof. Owen in the Cyclopædia of Anatomy and Physiology.
The conical teeth are extremely variable in size and form; some are slender, almost invisible points, distributed like the pile of velvet (villous-teeth), or set like the hairs of a brush (brush-teeth); some are long and slender, or barbed at the point; others are obtuse; and many are long and striated at the base, and closely resemble the teeth of certain reptiles. The depressed teeth are equally diversified; some have the grinding surface smooth; others, deeply grooved; in some it is flat; in others convex. In form they are either lozenge-shaped, elliptical, square, oblong, semilunar, &c. The cylindrical teeth are hemispherical, or flattened; in some fishes they are short and thick; in others slender and support an obtuse, conical crown. The prismatic form is equally modified; from the compressed, sharp, lanceolate, cutting teeth, to the strong, triangular, three-pointed dentary organs.[509]
[509] For illustrations of the teeth of fishes, see plate iv. figs. 1, 2, 8, 10, and Ligns. 189, 191-194, 197, 198, 202, 205; _Foss. Brit. Mus._ p. 449; and Ly. figs. 236, 308, 324, and 383.
The mode of arrangement and attachment of the teeth, is as diversified as their forms. In some species all the teeth are of one type, and disposed in somewhat of a serial order on both sides of the jaws; but in a large proportion of fishes there are several kinds of teeth, which are implanted not only in the jaws, properly so called, but on the bones which form the cavity of the mouth, the arches of the palate, tongue, &c.; and it is peculiar to this class of vertebrata to present examples of teeth developed in the median line (along the middle) of the mouth, as in certain species of Rays; or crossing the symphysis (the front line of union of the two sides) of the lower jaw, as in Myliobates[510] (see Lign. 194, fig. 2). In some species the teeth are implanted in sockets, to which they are attached only by the soft parts, as in the rostral teeth of the Saw-fish; some have hollow bases, supported upon bony prominences, which rise from the base of the socket; as in several fossil teeth from the Chalk. "But by far the most common mode of attachment is by a continuous ossification between the dental pulp and the jaw,"[511] the teeth being thus anchylosed to the bone. In the Sharks the osseous bases of the teeth are attached by a ligamentous substance to the tough, dense crust, which covers the cartilaginous jaws; the teeth of these fishes are therefore generally found detached in a fossil state, in consequence of the decomposition of this substance.
[510] Odontography, p. 5.
[511] Op. cit. p. 6.
The teeth are composed of a dense, osseous material, of a finely tubular structure, termed dentine; which, in many species, forms on the external surface of the tooth a layer of firmer texture, with a glossy surface, resembling enamel. The essential character of their organization is to have a pulp or medullary cavity, or cavities, filled with a plexus of blood-vessels and nerves, from which the minute tubes composing the dentine radiate.[512] The differences observable in the size, mode of ramification, and distribution of the medullary cavities or canals, and the calcigerous tubes,[513] as revealed by microscopic exploration, constitute important distinctive characters; particularly in the examination of the fossil teeth of extinct fishes. In some teeth the dentine is traversed by equidistant, parallel, medullary canals; in others, these channels frequently subdivide, and their branches anastomose with each other. In some the medullary canals form a reticulated, or net-like structure in the dentine, the meshes of which are occupied by calcigerous tubes, and cells; often producing a dendritical appearance, as in the tooth of a fossil fish named Dendrodus. "In the highest type of structure, the dentine consists of a simple medullary cavity or canal, and a single system of calcigerous tubes, which radiate from the central or sub-central pulp-cavity, at right angles to the periphery of the tooth" (Owen), as in the teeth of the extinct Sauroid (lizard-like) fishes. A continued succession of teeth takes place during the life of the fish, and we often find in fossil specimens a series of successional teeth beneath the row in use; as in the fragment of a jaw of Lepidotus, from Tilgate Forest Lign. 107.
[512] See Owen’s Odontography; and Tome’s Dental Physiology.
[513] Calagerous tubes; so named because they are composed of calx, or lime.
Skeletons of Fishes.--The skeletons of the animals of this class differ so remarkably in their relative degree of firmness and elasticity, in consequence of peculiar modifications of their constituent substance, as to form two grand divisions; one of which is termed the osseous, the other the cartilaginous. The essential difference in the skeletons of these two groups consists in the presence or absence of earthy matter (phosphate and carbonate of lime) in the materials of which they are constructed. In the cartilaginous fishes, the skeleton is cartilaginous and transparent; but in some species, the skin has dense osseous particles or plates on the skin, as in the Rays; and in others, the head and body are protected by large osseous scutcheons, as in the Sturgeon. There is also an intermediate group of fishes, termed the fibro-cartilaginous, in which the skeleton contains lime, but in a much less proportion than in the true osseous fishes. In some genera, certain portions of the skeleton, as the bodies of the vertebra, are cartilaginous, while the spinous processes, ribs, &c. are osseous; these characters are of considerable importance in the investigation of the fossil remains of fishes, as we shall hereafter have occasion to demonstrate.
The skeleton consists of the cranium or skull, which is composed of numerous bones,--the jaws, and bones of the tongue,--the osseous frame-work of the organs of respiration, consisting of the bones, rays, and arches that support the gills, and the opercula, or covers which close over the branchial apertures,--and of the vertebral column, formed of numerous dorsal and caudal vertebræ, with the ribs and other appendages; there are no proper cervical vertebræ, or spinal bones of the neck.
The branchial arches are in general four or five on each side, and are attached above to the cranium, and below to a chain of small bones, by which they are connected with the os hyoides, or bone of the tongue. The opercular bones, composing the cover or lid of the opening of the gills, consist of three pieces on each side, and are distinguished by the names, opercular, pre-opercular, and sub-opercular, according to the situations which they respectively occupy.
The vertebræ are double hollow cones,[514] not unlike an hour-glass in form: the interval between two of these bones is filled up, in the living state, by a gelatinous fluid. Along the upper part of each vertebra, there is an annular cavity, which in the united vertebral column forms a canal for the spinal marrow; the posterior dorsal and caudal vertebæ have also a channel below, for the passage of the large blood-vessels.
[514] There are certain exceptions; thus in the Lepidosteus the vertebral column is a series of ball-and-socket joints, the convexity being anterior, as in the land Salamander, and in the fossil reptile known as the Streptospondylus.
There are likewise bones analogous to some of those which enter into the composition of the extremities, chest or thorax, and pelvis of the higher vertebrata; but which it is not necessary for our present purpose here to describe.
Of the organs of vision some fossil remains also occur. The sclerotic coat, or capsule of the eye, being bony in fishes, is often preserved; and in several chalk specimens I have found it occupying the orbit.
In addition to those durable parts of fishes, already mentioned, as likely to be met with in a fossil state, the bones called otolithes (ear-stones) must be enumerated. These calcareous bodies are found in the membranous labyrinth of the organs of hearing; and, although more or less developed in the ear-bulb of all animals, they are larger and of more definite forms in the higher osseous and cartilaginous fishes. The otolithes are supposed to assist in communicating more vivid impressions of sounds to the extremities of the auditory nerves; they are stony in most aquatic animals, and friable or pulverulent in those that live on land. Smooth, oblong otolithes are not uncommon in the Crag deposits of Norfolk and Suffolk; and minute ear-bones are found in the Barton Clay.
Tails of Fishes.--The tail, as we have previously mentioned, is the chief instrument of progressive motion in these animals; it assumes two principal modifications. In the greater number of the existing species the vertebral column terminates in a triangular plate of bone (formed by the fusion of the last few vertebræ), to which the caudal fin is attached symmetrically; and its figure is either rounded, or divided into two equal lobes or branches; these tails are termed homocercal, i. e. even-tail. In the second modification the vertebral column towards its extremity diverges from a straight line, rises up, and is prolonged into the upper lobe of the tail; the caudal fin appearing like a rudder, and its low’er lobe, being destitute of vertebræ, is proportionably very feeble and small, as in the Shark and Dog-fish: this form of tail is called heterocercal, i. e. unequal-tail (see Foss. Brit. Mus. p. 421; and Ly. figs. 340, 341). In the embryonic state the tail in all fishes is heterocercal, and it becomes homocercal in the progress of development in those genera which have this type of the caudal appendage. But few of the existing species have the heterocercal tail, while it is found in all the fossil fishes that occur in the ancient secondary strata; namely, the Magnesian limestone, and antecedent deposits. The rounded and equal-bilobed, or homocercal, tails, are seen in the fishes from the Chalk, Wond. pp. 347, &c.; and in the Wealden Lepidotus, Lign. 186; and the unequal or heterocercal tail is shown in the Amblypterus from the Carboniferous strata, Lign. 187.
In the Annals of Nat. Hist, for 1848, p. 304, Prof. M’Coy has described and figured an intermediate form of tail, which he regards as characteristic of the Diplopterus (of the Old Red Sandstone) and its allies: this the Professor terms the Diphycercal tail.
From this brief summary of the essential characters of those durable parts of the organization of fishes, which most frequently occur in a fossil state, we pass to the investigation of some illustrative examples of this class of organic remains. But before describing any entire specimens, it will be expedient to notice the separate fins, and teeth, which abound in many deposits; in some instances occurring in connexion with other parts of the skeleton, but more generally detached, and constituting the only evidence of the existence of numerous extinct species and genera. The greater part belong to the first order--the Placoidians (Poiss. Foss. tom. iii.), and to the families of Sharks and Rays. The osseous dorsal rays of cartilaginous fishes (named Ichthyodorulites (fossil-fish-weapons) by Dr. Buckland and Sir H. De la Beche) first demand our notice.
Ichthyodorulites. Lign. 188.--This name is applied to the fossil spines, or rays, of dorsal fins, of which numerous species occur in the Secondary deposits; they belong, for the most part, to extinct cartilaginous fishes of the Cestracionidæ and Hybodontidæ groups. In the osseous tribes the dorsal spines have at their base two articular processes, by which they are united to the osselets that support them, as in the Silurus; but in the cartilaginous, they have no articulations at the base, and terminate in an obtuse point, which is implanted in the flesh; the posterior margin is grooved almost to the upper extremity. They are of a fibrous, osseous texture. The common Spinax, or Dog-fish (Acanthias vulgaris), has a spine of this kind in the front of each dorsal fin. The rays of the Sharks are compressed, and some have rows of teeth on the posterior margin; in the genus Cestracion (Port-Jackson Shark), these organs are strong, triangular, straight, pointed, rounded in front, flat at the posterior face, and widest at the base; in the Hays they are flattened or depressed.
These spines are generally capable of being elevated and depressed, and not only serve the purpose of defence, but, in many instances, afford support and protection to the soft rays of the fin; forming, as it were, a moveable mast, by which the sail can be spread out or lowered at pleasure.
In illustration of this subject, I would first direct attention to the beautiful fossil, figured Lign. 188, fig. 1, which was discovered in the Chalk near Lewes, and is figured, of the natural size, Foss. South D. tab. xxxix. This ray, or spine, belongs to one of the Cestraciont fishes (Ptychodus), whose teeth are so abundant in the Chalk, and will presently be described. It is composed of fourteen thick, flat, osseous rods, or strands, intimately united together, with longitudinal furrows or sutures on the surface. The anterior margin is embossed, and the projections form on the sides wide, rounded ribs, and transverse depressions. Towards the base of the posterior part, there are large osseous fibres inserted vertically and obliquely, which appear to have been processes of attachment. The rods, or plates, are parallel With the posterior margin, and each terminates in a rounded extremity, or boss, on the front edge of the spine. This ray is wider at its base than at the superior part the anterior margin is oblique, and the posterior straight. The surface, where entire, is covered with a dense osseous substance, which is finely engrailed.[515]
[515] This specimen is figured in Poiss. Foss.; but it is represented too short, from the two portions being drawn as if they were connected, without any interval between them, as in Lign. 188. It is in the British Museum. See Petrifactions, p. 450.
Fig. 1.--Ptychodus spectabilis. 1/5 nat. Chalk. Lewes. 1_a_.--Portion of a Dorsal Ray, with oblique serrated sutures. Chalk. Lewes. 2.--Ptychodus Gibberulus. 1/5 nat. Chalk. Lewes. 3.--Hybodus subcarinatus. nat. Weald. Tilgate Forest.
In 1850 I discovered in the Plastic Clay of Castle Hill Newhaven, a dorsal fin of Ptychodus, with eight vertebræ. A nearly entire fin-ray of this species, three feet in length, has recently been discovered by Mr. Charles Potter, of Lewes, in the Chalk near that town. The remains of another ray, of equal proportions, were found near it; and these dorsal spines might have belonged to the same individual, for there are no reasons to forbid the supposition that the Ptychodus had two dorsal fin-rays. The length of these spines necessarily indicates a very large fish.
A smaller species of Ichthyodorulite, also found in the Lewes Chalk, is distinguished from P. spectabilis by its osseous plates contracting towards their extremities, and terminating more suddenly on the front margin, producing gibbosities less acute and more distant than in P. spectabilis; this species is named P. gibberulus: see Lign. 188, fig. 2.[516]
[516] This fossil is figured of the natural size, Foss. South D. pl. xl. fig. 3.
The bony plates of these fins are occasionally found lying in irregular groups in the Chalk, as if the fin had partially decomposed and the plates separated. In one example, the rays are split asunder by a piece of bone, apparently a portion of a long pointed tooth, firmly impacted between them; as if the fish had been seized by some enemy, and had escaped, with the tooth of its adversary in its fin. Very fine specimens have been found at Charing, Kent, by W. Harris, Esq. F.G.S.
In the fragment of an Ichthyodorulite from the Lewes Chalk, a remarkable structure is displayed; the osseous plates are united laterally by smooth, longitudinal lines, as in those above described; but they are also traversed by numerous oblique, finely-serrated sutures. Lign. 188, fig. 1a.
The Chalk contains rays of other species of Ptychodus, as well as of some allied genera. Of these, the most remarkable are smooth, arched, pointed spines, having a shallow posterior groove, with an enamelled surface, marked with fine longitudinal striæ, and frequent, parallel, oblique lines. These, according to Sir P. Egerton, belong to a true Cestracion (see p. 584): they were first figured and described by me (Foss. South D. tab. xxxiii. fig. 5) as belonging to the Acanthias major, and were subsequently assigned to the genus Spinax by Prof. Agassiz (Poiss. Foss. iii. p. 62).
It may be necessary to remark, that the fins first described have been referred to the fishes which yielded the large grooved teeth so common in the Chalk (see Pl. VI fig. 2) in consequence of their affinity to existing species, which have similar fins and teeth; and from the circumstance that the Sharks of the genus Lamna, whose teeth also abound in the Chalk, have no dorsal rays of this kind; still the proof of identity remains to be discovered. In one specimen only have I observed indications of any other part of the skeleton; it is a spine of Acanthias major, the base of which rests on several dorsal vertebræ (Foss. South D. tab. xxxiii.).
Hybodus subcarinatus. Lign. 188, fig. 3.--The fishes of another extinct genus of Sharks, termed Hybodus, from the gibbous form of the teeth, were also provided with dorsal spines, which may be readily distinguished from the preceding. These Ichthyodorulites are generally long, slightly arched, and terminate in a point at the extremity; the base, which was implanted in the flesh, is deeply grooved, and much prolonged, being sometimes equal to one-third of the entire length. The surface is marked with strong longitudinal ridges, parallel with the anterior margin which is rounded and laterally compressed. The posterior edge, which is more or less flat, has, towards the base two rows of sharp arched teeth, which gradually approach ’each other, and blend into one line on the upper part of the ray There are numerous species of this genus in the Oolite and Lias. I have found one species in the Chalk and a few in the Wealden. The small Ray figured Lign. 188 fig. 3, is from Tilgate Forest, and displays the usual characters of these fossils. From specimens discovered in the Lias, associated with the teeth, it appears that the Hybodus had two dorsal fins, each furnished with rays, as in the recent Dog-fish.
The microscopic structure of these rays is stated by M. Agassiz to resemble that of the teeth: in some there is a pulp cavity, which occupies the centre of the spine, and is surrounded by dentine, in which the calcigerous tubes radiate direct to the surface; the external enamel is a layer of dentine, in which the medullary canals are wanting.
In the strata below the Lias there are numerous Ichthyodorulites, some of a large size, belonging chiefly to the Cestracion family, and of extinct species, not observed in more recent deposits. Thus there are several species of dorsal rays (named Onchus, from their hooked form,) that are wide at the base, and bent backwards, with the posterior margin destitute of teeth, in the Carboniferous, Devonian, and Silurian formations; also immense compressed spines, having small teeth on the posterior margin, and the surface covered with longitudinal striæ, and finely toothed, transversely; hence termed Ctenacanthus, or pectinated-spine (Murch. Sil. Syst. p. 596).
The fossil spine, named Orthacanthus (Poiss. Foss. vol. iii. pl. xlv.), and found in the Coal of Manchester, has been discovered in connexion with the body of the fish to which it belonged in the Carboniferous deposits of Ruppersdorf in Bohemia (Geol. Journal, vol. v. part ii. p. 23).
Some Ichthyodorulites have the surface richly ornamented with stellular tubercles, and are termed Asteracanthus, or starry-spine; there are very large fin-rays of this kind in the Wealden, Purbeck, Oolites, and Lias.[517]
[517] For particular information on Ichthyodorulites, consult Poiss. Foss. tom. iii. chap. i. About seventy species are enumerated.
The Ichthyodorulites of the Rays have no cavity like those of the Sharks, and are of a depressed form, and more or less flattened; they are armed with teeth along their exterior margins, and not on the posterior edge, as in the latter family.
Fossil Teeth of Fishes.--From the durable nature and striking appearance of many of the fossil teeth of fishes, and their prodigious numbers in some deposits, they are familiar objects to the collector. By far the largest proportion of the detached teeth belongs to various species and genera of that most numerous, and widely distributed family of voracious fishes, the Sharks. In the Tertiary strata teeth of this kind occur of a very large size; in the Chalk many species abound, particularly of the lanceolate and compressed forms, and of the rugous, mammillated, palatal teeth, commonly termed palates. As we pass to the more ancient formations, teeth of different forms prevail; and those which approach the recent types are either very rare or altogether absent. We will select some examples of the different genera in illustration of this subject; the previous observations on the form and structure of the recent teeth render but few introductory remarks necessary.
Fossil Teeth of Sharks.--The fishes of the Shark and Ray families belong to the Placoid order; the scales in the former consist of enamelled plates and tubercles, forming a shagreen surface; and in the dermal integument of the latter they appear as spines and bosses, irregularly disposed.
Notwithstanding the diversity in appearance of the teeth of Sharks, they all possess one essential character of structure, namely, a base, or osseous root of variable form, which is implanted in the integuments; and a crown, or external portion, which projects into the mouth, is covered with enamel or compact dentine, and assumes numerous modifications, by which the fossil genera are characterized. These teeth are never imbedded in sockets, nor united to the dentary margins of the jaws; they only adhere to the integuments of the mouth, and the covering of the maxillæ; they possess, in most of the Sharks, great mobility. They are generally disposed in rows; the anterior ones, being first used, fall out, and are replaced by those on the inner series. New teeth are also continually formed behind those in use, and advance successively towards the anterior rows as the latter are shed, and in their turn occupy the front rank. (See Cyclop. Anat. Art. Teeth.) An examination of the fossil and recent teeth of Sharks and Rays proves that the prevailing existing generic types have but few, if any, representatives in the fossils, except in those which belong to the Tertiary and Cretaceous formations; while the genera that appear isolated, as it were, in the present seas have numerous analogues in the Secondary strata.
The fossil teeth of this family may be divided into two grand divisions; namely, those which are more or less of a polygonal, obtusely conical, or depressed form, having a tesselated arrangement in the mouth; and those of a triangular, lanceolate shape, with cutting, or serrated edges, disposed in a series of rows on the jaws. The teeth of the first group (Cestracionidæ) have most analogy to those of the living genus Cestracion (Port-Jackson Shark); the second (Sgualidæ) to the Sharks, commonly so called.
The Cestracion is the only living representative of a family of squaloid fishes of a peculiar type, whose remains occur in almost the earliest fossiliferous deposits; it inhabits the seas of New Holland and the southern coasts of China. The jaws of the Cestracion are relatively very large, and are armed with numerous rows of teeth, essentially of two kinds; those situated anteriorly, or towards the front of the mouth, being adapted for seizing and retaining the food, and the posterior ones for crushing and bruising. The prehensile teeth are sharp, angular, and pointed: the others are obtuse, polygonal, enamelled, and disposed in oblique rows along the margins and inner surface of both jaws; there are sometimes sixty in each jaw (see Bd. ii. pl. xxvii 11. fig. A). Fossil teeth of this type are exceedingly numerous in the Chalk, Lias, &c. but are very seldom found in juxtaposition; the decomposition of the cartilaginous integuments in which they are imbedded, having, in most examples, occasioned their displacement and dispersion; specimens, however, are occasionally discovered, in which numerous teeth, of various sizes, are disposed in mosaic, in their natural relative positions.
The extinct forms of this family (Cestracionidæ) are known almost only by their teeth; and according to the shape, structure, and sculpture of these organs, M. Agassiz has arranged them into several genera. They occur in most of the fossiliferous deposits.
Cestracion canaliculatus.--The teeth of a fish belonging to the existing genus have been discovered in the Chalk of Kent; they are figured and described by Sir P. Egerton in the beautiful work by Mr. Dixon.[518] This unique specimen consists of a group of thirteen posterior molar and three or four detached prehensile anterior teeth, imbedded in a block of chalk about two inches square. The chief distinction from the teeth of the recent Cestracion is in the presence of a large medullary canal which traverses the base of each tooth: hence the specific name.
[518] Dixon’s Fossils of Sussex, &c. p. 365, tab. xxxii. fig. 8. From the t examination of a specimen lately found at Lewes, Sir P. Egerton has been led to assign to this species the spine formerly described as Spinax major.
Acrodus (ridge-tooth) nobilis. Lign. 189, fig. 4, Ly. p. 275, fig. 307.--In the Lias and Oolite, oblong enamelled teeth, having the surface of the crown covered with fine radiating grooves and striæ, are well known to collectors, in many parts of England, by the name of fossil leeches, from a fancied resemblance to a contracted leech. They belong to an extinct genus of Cestracionts, named Acrodus by M. Agassiz. The crown of the tooth is enamelled, and covered with transverse grooves, which diverge from a longitudinal furrow; the base is in the form of a parallelogram inclined on its inner side. These teeth were inserted along the jaws in oblique series, their longitudinal direction corresponding with that of the bones which supported them; in their natural position, the extremity of a hinder tooth was enclosed between the two next anterior teeth. A beautiful group is figured Bd. ii. pl. xxvii^e.[519]
[519] The microscopical structure of the teeth of Acrodus is well shown in the "Odontography," pl. xiv. xv., and beautifully illustrates the relation of dentine to bone.
Fig. 1.--Ptychodus Mortoni. (G. A. M.) Cret. New Jersey. 2.--Psammodus cinctus. (Ag.) Mt. L. Bristol. 3.--Orodus cinctus. (Ag.) Mt. L. Bristol. 4.--Acrodus nobilis. Lias. Lyme Regis.
Ptychodus (wrinkle-tooth). Pl. VI. fig. 2; Lign. 189, and Lign. 191.--The palatal teeth, which occur more or less abundantly in almost every chalk-pit, and are known by the name of "palates," belong to several species of the genus Ptychodus. A very common form is figured Pl. VI. fig. 2; and microscopic views of vertical and transverse sections, as seen by transmitted light, are shown in figs. 2^b, 2^c. Groups of these teeth, somewhat naturally arranged, and varying in size and form according to the situations they occupied in the jaws, are occasionally found: one specimen in the British Museum, and formerly in my collection, contains more than 120 teeth. In general they occur in a very perfect state, with the osseous base and enamelled crown entire. The dorsal rays or spines previously described (p. 577), are sometimes found with the teeth, and belong to fishes of the same genus.
These teeth are of an angular form, and more or less square, the crown is wider than the root, which is obtuse, truncated, and depressed in the centre; the enamelled part of the tooth is expanded at the edges, and forms in the centre a flattened or slightly convex mammillary projection, which is traversed by large, acute, transverse, parallel ridges. The borders are granulated, and the sides of the projection marked with deep vertical plicæ or folds; this description particularly applies to the species named P. polygurus, figured in Plate VI. Dr. Buckland has represented a fine group of these teeth, Bd. ii. pl. xxvi′. Another common species (P. decurrens) is distinguished from the former by the connexion between the large furrows on the crown and the granulations on the expanded border, which diverge from the outer edge of the large folds to the margin of the enamel.
The microscopic structure of these teeth presents the same congeries of medullary and calcigerous tubes as those of the recent Cestracion: see Plate VI. figs. 2^b, 2^c.
The teeth of a species of Ptychodus occur in the arenaceous strata of the Chalk-formation in New Jersey, which possess the essential characters of the European types, but differ from them in their configuration; the only specimen I have seen is figured Lign. 189, fig. 1; it was presented to me by Dr. Morton. The enamelled crown forms a conical projection, traversed by large inosculating ridges, which radiate from the summit towards the margin.[520]
[520] I have named it P. Mortoni, in honour of my distinguished friend, the eminent American naturalist and physician, Dr. George Morton, of Philadelphia, by whom it was discovered.
Psammodus[521] (sandy-tooth). Pl. VI. fig. 1; Lign. 128, fig. 2.--To this genus are referred the fossil teeth of the extinct Cestracionts, which have the crown formed of small vertical tubes, with the grinding surface more or less smooth, and presenting only a punctated or sandy appearance.[522] These teeth are generally flat or slightly convex, and of a square or oblong form; the base is osseous, and as large as the crown. Two species are figured, Lign. 189, fig. 2, and Pl. VI. fig. 1^a. A magnified vertical section of the crown, displaying the medullary canals and radiating calcigerous tubes, is represented Pl. VI. fig. 1^b, and a transverse section, fig. 1^c; they are thin slices of a tooth, P. porosus, from the Black Rock (Mountain Limestone), near Clifton, viewed by transmitted light. The large, flat, quadrilateral, oblong teeth that abound in the Stonesfield Slate belong to the Strophodus magnus.
[521] See Odontography, pl. xviii. xix.
[522] Ibid. vol. i. p. 59.
There are several kinds of fossil teeth which possess the same essential structure as those of Psammodus, but differ in their external characters; these are referred to other genera by M. Agassiz. Thus Orodus, Lign. 189, fig. 3, comprises those elongated teeth in which the centre of the crown forms an obtuse transverse cone, traversed by a ridge from which oblique furrows diverge transversely towards the circumference. Similar teeth, but with a smooth, obtusely conical crown, are referred to the genus Helodus. Those with the crown compressed and elevated, with a sharp edge, and with the base surrounded by concentric folds, constitute the type of Chomatodes. A similar crown, but subdivided by deep transverse ridges into dentations, characterises the genus Ctenoptychius.
Ceratodus (horn-tooth) emarginatus. Lign. 194, fig. 1.--Very curious dental organs, possessing a structure analogous to that of the teeth of Psammodus, are found in the Bone-bed of the Lias; they consist of consolidated plates instead of separate teeth; there was probably but one plate on each side the jaws. The upper margin is generally undulated, and more or less worn by use. These dental plates are composed of two distinct layers; the lowermost portion, or root, is an osseous, reticulated tissue, as in cartilaginous fishes in general; and the upper consists of dentine, with minute parallel vertical tubes, as in Psammodus; these tubes are a continuation of the medullary tissue of the osseous root.
One species occurs in the Great Oolite at Stonesfield, and very many forms abound in the Bone-bed at Aust Cliff, near Westbury on Avon: and in the Trias (bone-bed) of Germany the teeth of several species of Ceratodus are very abundant.
The fishes to which these fossil teeth, referred to Ceratodus, belonged were most probably Cestracionts; the ray-spine known as Nemacanthus is provisionally assigned to them.
Edaphodon. Lign. 190 and Lign. 191, Ly. p. 276, fig. 309.--The Chimæroid fishes, though formerly placed with the Plagiostomes (Sharks and Bays), constitute a distinct group, of which there are but two recent genera, though several occur in a fossil state. Their dental organs are very peculiar. Their mandibles are furnished with two or more pairs of oblong teeth, composed of long hollow cylindrical columns, placed nearly at right angles to the grinding surface, which is pitted with minute depressions. These teeth are never shed, but are persistent, and grow on through life, as in the Rodentia, exhibiting in this respect a striking contrast with those of the Sharks, which are feeble and numerous, and constantly replaced by rows of successional teeth.
Fossil teeth of several species, some much larger than the recent, have been found in the Tertiary, Cretaceous, and Oolitic deposits. The first British specimen was discovered in the Chalk-marl at Hamsey, in 1820, by myself; but its nature was not suspected until more perfect examples were obtained from the Kimmeridge Clay at Shotover by Sir P. Egerton, and were submitted to Dr. Buckland, who subsequently ascertained their characters and relations by an examination of the dental organs of a recent Chimæra in the Museum at Leyden in 1835.[523]
[523] Proceedings of the Geological Society of London, vol. ii. p. 209.
Many specimens, both of the upper and lower mandibles, have since been discovered in the Eocene beds, Chalk, Upper Greensand, Galt, Kimmeridge Clay, and Stonesfield Slate. The subject has been carefully investigated by Sir P. Egerton; and this eminent Ichthyologist has tabulated the principal forms, and arranged them under five genera.[524]
[524] Viz. Ganodus, Ischyodus, Edaphodon, Elasmodus, and Psaliodus. See Quart. Journ. Geol. Soc. vol. iii. p. 35; and Dixon’s Foss. Sussex.
In some species the external vertical wall of the plate is formed of hard dentine, resembling enamel; in others the dentine is disposed in isolated ramifications, producing a dendritical appearance; the modifications of this structure occasion the differences observable in the dental plates of the various species. In some, compact dentine with parallel canals constitutes the mass of the tooth; in others, the squamous dentine with ramifying tubes prevails.
I have figured the right upper and lower mandibles of the type named Edaphodon (pavement-tooth), in which there are three teeth or dental tubercles on each ramus of both jaws, Lign. 191: the lower mandible is produced anteriorly into a falciform beak:[525] the articulating surface of the symphysis (2 s) is broad at the base, and contracts gradually forward until the margins meet at the apex. In Ischyodus the lower jaw is deeper, less produced in front, and the margins of the symphysis are parallel until abruptly truncated at the extremities: the upper jaw has four tubercles on each side.
[525] Hence M. Agassiz proposed the name of Psittacodon (parrot-tooth) for this genus of Chimæroids.
The upper jaw in Elasmodus has but three tubercles, as in Edaphodon, but the dentine of which they are composed is confluent, being rolled round like a scroll in the substance of the bone, one edge forming the margin of the tooth, and the other being buried deep in its centre.
The dorsal fin-ray or spine of the Edaphodon is laterally compressed, with the posterior margin grooved, and the edges armed with fine teeth: I have a specimen of the spine, with a pair of inferior mandibles of the same individual, imbedded in a block of chalk from Kent; by favour of Mrs. Smith, of Tunbridge Wells.
Hybodus. Lign. 192, figs. 1, 2. (Bd. pl. xxvii^d.)--Intermediate between the obtuse crushing teeth of the Sharks previously described, and those sharp, angular, pointed, dental organs of the Squaloids, are those of the fishes which M. Agassiz has arranged in a sub-family or group termed Hybodonts; the teeth of which are characterised by their transversely elongated form, and the series of subacute, compressed, conical cusps or points, which compose the crown. The median cone is the principal, the lateral points being shorter and smaller, as in Lign. 192, fig. 2; in some species the difference between the median and lateral cones is greater, in others less, as in fig. 1. These cusps have a coating of dense enamel, which is plicated longitudinally on both faces. The base, which almost equals the crown in size, is composed of a coarse osseous substance. The internal structure of the crown differs from that of the Cestracionts, in having no principal pulp-cavity, and in being chiefly composed of dendritical dentine, with reticulated medullary canals. The form and organization of these teeth show them to have been instruments for cutting and tearing food. The Hybodonts, as we have already stated (p. 581), possessed two spinous dorsal fins; in their habits and economy they probably did not differ from the ordinary Sharks. Teeth and spines of this genus are common in the Trias, Lias, Oolite, and Walden, and occur in the Green Sand and Chalk. There are several species of teeth and fins in the strata of Tilgate Forest (Foss. Til. For. pl. x.). In general the teeth are found detached, but occasionally they occur in their natural position, adhering to the mineralized cartilaginous jaws (Petrif. Lign. 97); as in the beautiful fossil figured Bd. pl. xxvii.^d; and in the fine specimen of H. basanus, from the Isle of Wight, figured in the Geol. Soc. Journ. vol. pl. iv. There are several related genera, founded on the situation, form, and division of the principal cusps of the teeth.
Sharks with Cutting Teeth.--The jaws of the common squaloid fishes, as the Lamna (Porbeagle) and Carcharias (Great White Shark), are so common in collections of natural history, as to render a description unnecessary. The numerous vertical rows of angular, laterally compressed, pointed teeth, with sharp or serrated edges--in some species consisting of a simple trenchant cusp, in others with small lateral teeth, or denticles, at the base, are characters with which all are familiar. Fossil teeth of this form are extremely abundant in the Tertiary and Cretaceous deposits; and are commonly in a beautiful state of preservation. The genera of these fossil teeth are founded on the solidity or hollow structure of the cusps, their possessing cutting or serrated edges, and the presence or absence of lateral denticles. But the last character cannot in every instance be relied upon, for some recent Sharks have rows of teeth both with and without denticles.
Carcharodon productus. Lign. 192, fig 3.--The genus Carcharias comprises the large Sharks with cutting triangular teeth, crenated (notched) on their margins, and having a broad base. In Carcharodon, the teeth differ from those of Carcharias in being solid in the centre, while in the latter they are hollow; but in both genera the teeth exhibit the same reticulated structure of medullary and calcigerous tubes. The White Shark and other large species belong to these genera; some of which are upwards of forty feet in length. But even these colossal fishes must have been far surpassed in magnitude by the extinct species of the Tertiary deposits, if the teeth afford a scale of proportions; for some of the fossil teeth from Malta and the United States are six inches long, and five wide at the base;[526] being twice the size of the teeth in the largest living species. The specimen figured in illustration, Lign. 192, fig. 3, is of a small size.
[526] For instance, some of the Maryland specimens of Carcharodon megalodon. See an admirable memoir on the Fossil Squalidæ of the United States, by Dr. R. W. Gibbes, Journ. Acad. Nat. Science, Philadelphia. At the meeting of the British Association in 1851, J. S. Bowerbank, Esq. F.R.S. read some interesting observations on the comparison of these large fossil fishes with the recent Carcharias glaucus of Australia. See also Owen, Cyclop. Anat. Art. Teeth.
Hemipristis serra. Lign. 192, fig 4.--The fossil teeth of this genus are distinguished by serrated edges, that do not extend to the summit, which is a sharp angular point; as in the fossil represented.
Lamna elegans. Lign. 193, fig. 6.--The fishes of the genus Lamna (to which the recent shark called the Porbeagle belongs) have teeth with smooth trenchant edges, and a small sharp denticle (little tooth) on each side the base, as in the fossil, Lign. 193, fig. 6. The specimen, fig. 2, although devoid of denticles, probably belongs to the same genus, for reasons already explained. Several species abound in the Chalk; and they are associated with teeth, which are relatively wider and shorter, and have large compressed denticles; the latter are arranged in a separate genus, named Otodus (eared-tooth), by M. Agassiz. The specimen figured Lign. 192, fig. 5, represents O. obliquus; another species, Otodus appendiculatus, is abundant in the Sussex Chalk. The large, wide, triangular, smooth teeth, with trenchant edges, and destitute of lateral denticles, so common in the Chalk, are related to Lamna, and are comprised in the genus Oxyrhina (Poiss. Foss. tom. v. tab. xxxiii.).
Notidanus microdon. Lign. 193, fig. 3.--These teeth differ remarkably from those of the other genera of Sharks. The crown of each tooth is composed of a series of sharp angular enamelled points, the first of which is the largest, and is notched on its anterior edge; the base or root is osseous, flat, with a slight longitudinal depression below the border of enamel. These teeth are comparatively rare in the Chalk. One species has been found in the Oxford Clay; and several in the Tertiary strata. Specimens occur in Hordwell Cliff.
Corax pristodontus. Lign. 193, fig. 1.--The teeth of the fossil Corax chiefly differ from those of the recent genus Galeus, to which the Tope, or Grey Shark, belongs, in being solid; they are of a triangular form, with a deep concavity or notch on the posterior margin, the base of which is prolonged and forms three or four angular points: the anterior edge of the tooth is finely serrated. The root of the tooth, as in Notidanus, is a broad osseous plate. There is much diversity of form in the Chalk specimens, which are all of a small size, as in Lign. 193, fig. 1. In Sussex they are more common in the Chalk-marl than in the Chalk.
The only fossil teeth of the Shark family resembling those of the tertiary Carchariodonts, that have been discovered in the strata below the Chalk, are from the carboniferous deposits of Yorkshire and Armagh. These teeth are compressed, triangular, crenated on the edges, with large plaits or folds on the enamelled surface, towards the base of the crown. M. Agassiz refers them to a new genus, viz. Carcharopsis, with the specific name of Prototypus.
Fossil Vertebræ of Sharks.--As the cartilaginous nature of the skeleton in this family renders it unfavourable to preservation in the mineral kingdom, the durable parts already described, and those which are ossified, are almost the only relics found in a fossil state. The dermal integument is, however, sometimes preserved; and I had a beautiful example of shagreen, composed of irregular minute hexagonal scales, one of which is represented highly magnified, Lign. 185, fig. 1.
In the Galeus and Carcharias the vertebræ are more ossified than in many other genera of cartilaginous fishes, and fossil vertebræ of these sharks often occur in the cretaceous and other strata. Groups of vertebræ of a large size occasionally occur in the Sussex Chalk; they are circular, biconcave, and very short; one specimen is four inches in diameter, and one inch long; their concavities are consequently shallow. These vertebra: are composed of two shallow conical disks, which are united by their summits, at the axis, and are connected and supported by numerous wedge-shaped plates, that radiate from the centre to the periphery (see Foss. South D. pl. xxxiii. fig. 10). My collection contained a connected series of forty small vertebræ from the Chalk near Lewes, which probably belonged to the same species of Shark as the dorsal spine named Spinax major (Poiss. Foss. tom. iii. pl. xl^a fig. 6).
Squaloraia. In the Lias of Lyme Regis, that inexhaustible storehouse of fossil treasures, a considerable portion of the skeleton of a very remarkable fish, partaking of the characters of the Sharks and Rays, was discovered by Miss Mary Aiming, and is now in the Museum of the Bristol Institution.[527] In this fish the jaws are prolonged into a beak, like the Pristis (Saw-fish). It has the head of a Shark, with a long beak; vertebræ of the Rays; with pectoral and ventral fins, almost equally developed; a tail armed with, a spine; and spinous bosses, as in the true Rays.
[527] It is figured and described by Dr. Riley, Geol. Trans. 2d ser. vol. v. pl. iv. See also Poiss. Foss. tom. iii. pl. xlii.
Fossil Pristis, or Saw-fish.--This well-known predatory fish, which is allied to the Rays and Sharks, and referred by M. Agassiz to the family of Raiidæ, has projecting from its snout an osseous, flat, horizontal plate, or beak, equal in length to one-third of the fish, and armed on each side by a row of elongated, compressed, pointed teeth, implanted in sockets; the front margin of these teeth is convex, the posterior concave; this defence is termed the saw, and constitutes a most powerful weapon. The Pristis has also numerous small obtuse teeth on the jaws. The remains of the beak, or saw, of an extinct species of Pristis have been discovered in the Bagshot Sand at Goldsworth Hill, Surrey,[528] and three other species have been found in the London clay of the Isle of Sheppey, and the Eocene beds at Bracklesham and Hordwell.[529]
[528] Proc. Geol. Soc. vol. ii. p. 687.
[529] Two teeth are figured in Dixon’s Fossils of Sussex, pl. xii.; the specimens are in the British Museum; see Petrifactions, p. 414.Return to passage
Fossil Rays.--The teeth of these fishes are characterised by the extraordinary transversal development of the median teeth in both jaws. Instead of pointed teeth, they have wide, flat, tesselated dentary plates in each jaw, composed of distinct pieces, juxtaposed and connected by their margins, and united by fine sutures. In some species the teeth are equal, in others of various sizes; they present numerous modifications of arrangement, and are always disposed in symmetrical rows. In the genus Myliobatis (Eagle-ray) the teeth of the median row are of an extraordinary width, while their length does not exceed that of the lateral plates, or chevrons, which are of an irregular hexagonal form, and disposed in two or three rows on each side. There are five living species of Myliobatis, and eighteen fossil species have been found in the Tertiary strata at the Isle of Sheppey, Hordwell Cliff, and Bracklesham Bay. I have figured a specimen of part of the upper jaw of a species (M. micropleurus, Lign. 194, fig. 2), in which the median teeth are very wide, and have two lateral rows of small, irregularly hexagonal, plates. The surface of the teeth in this species is smooth; but in others it is striated longitudinally (Bd. pl. xxvi^d. fig. 14). In an allied genus, Ætobatis, from the Tertiary beds at Bracklesham, the lower jaw projects beyond the upper, and in each there is a row of flat, transverse teeth, without lateral plates.
To this notice of the fossil Rays, we may add, that a gigantic Torpedo has been discovered in that celebrated locality of Ichthyolites, Monte Bolca: and that Sir Philip Egerton has recently enriched his matchless collection of fossil fishes, by a perfect Ray, from Mount Lebanon. It is figured and described in the Quarterly Geol. Journ. vol. i. pl. v. p. 225, under the name of Cyclobatis oligodactylus. It is a small species, resembling the common Rays in its general appearance, but is surrounded by a broad, flexible, cartilagino-membranous fin; the skin is smooth, the teeth and eyes are small, the tail is slender; there are no traces of dermal spines, tubercles, or defensive weapons. In many points of structure it resembles the Torpedo; and may possibly, like that fish, have possessed an electric organ. It is 3-1/2 inches long, and 3 inches wide across the expanded fins.
Chapter XV
FOSSIL ICHTHYOLOGY; COMPRISING THE GANOID, CTENOID, AND CYCLOID FISHES.
The fishes we have hitherto examined belong to the first order, the Placoidians; we now pass to the fossil remains of the second order, the Ganoidians, which are distinguished by their brilliant angular scales, formed of osseous or horny plates, densely covered with enamel. This order contains six or more families, comprising many genera and numerous species; our investigation must be restricted to a selection from the principal genera of the Ganoids, properly so called, and of the Sauroids, or lizard-like fishes.[530]
[530] The fishes of these orders are described in Poiss. Foss. tom. ii.
The first family, termed Lepidoides, contains several genera, which are defined as possessing either numerous rows of brush-teeth, or of obtuse conical teeth; flat, rhomboidal scales, arranged parallel with the body; and an osseous, or partially osseous, skeleton. In one division of this family, the body is either elongated or fusiform, the mouth furnished with brash-teeth only, and the tail heterocercal, or unequally bilobed (see p. 576). To this group belong several genera, which are restricted to the Secondary formations more ancient than the Oolite; while the other group, with homocercal tails, lived in the Oolitic and Cretaceous seas. Two genera, in particular, abound in the Permian and Carboniferous strata; namely, Amblypterus and Palæoniscus.[531]
[531] For the characters, affinities, and distribution of these and the allied genera of the Heterocerque Ganoid fishes, see Sir P. Egerton’s Memoir in the sixth volume of the Quarterly Geological Journal.
Amblypterus. Lign. 187. Wond. p. 740. Bd. pl. xxvii.^b.--The fishes of this genus, as the name indicates, have very large and wide fins, composed of numerous rays. The scales are rhomboidal and finely enamelled; the tail is heterocercal. The figures referred to convey a correct idea of the form and external characters. Beautiful pyritous imprints of Amblypteri occur in the Carboniferous slate of Saarbrück, in Lorraine; and fine specimens in the ironstone nodules of the same locality. On the shore at Newhaven, near Leith, similar fossils occur in nodules washed out of the cliffs of coal-shale (Bd. p. 278).
Palæoniscus. Ly. p. 304.--The fishes of this genus differ from those of Amblypterus in the relatively moderate size of the scales, and the numerous little rays on their margins. They have rhomboidal scales, which in some species are very small, and in others large. They have numerous brush-teeth. Several peculiar species, found in the marl-slates and magnesian limestones of the Permian system, are very widely distributed, occurring in the British Isles, Germany, and the United States.
In some localities the small species occur in groups; on a slab of red sandstone, in the Museum of the Geological Society, from Tyrone, between two and three hundred perfect fishes (P. catopterus) are imbedded on a space not exceeding two feet square.
A remarkable circumstance relating to the fishes of this genus is the almost constant absence of the bodies of the vertebræ in otherwise well-preserved specimens, and in which the spinal processes and the ribs are entire: occasionally, however, examples occur with some of the vertebræ perfect. An explanation of the above phenomenon may perhaps be found in the probable originally cartilaginous nature of the bodies of the vertebræ, and the osseous structure of the enduring apophyses and ribs;[532] while those rare specimens which possess a few bony vertebræ may be regarded as exceptions, in which ossification took place in a structure essentially cartilaginous.
[532] Professor Owen states that a similar condition of the spinal column obtains in the fossil Microdonts.--Rep. Brit. Assoc. 1846.
The fishes found in the copper-schists of the Zechstein, at Mansfeld in Saxony, are generally impregnated with copper pyrites, and their scales are as brilliant as burnished gold. These ichthyolites are almost always in contorted and twisted positions; which appearance M. Agassiz attributes to contraction of the muscular tissues after death, during the progress of decomposition, and before the fishes sank down and became imbedded in the mud. (Poiss. Foss. tom. ii. p. 70.)
The fishes of the genus Palæoniscus are often found in the shales and marls of the Permian and Carboniferous systems of England and Scotland. At East Thickley, in the county of Durham, numerous specimens have been found.[533] The lower Carboniferous strata at Burdie-house, a locality we have before mentioned, have yielded several species of Palæoniscus, associated with teeth and other remains of large sauroid fishes.[534] On the continent also they prevail in deposits of the same epoch; Eisleben and Mansfeld, iii Saxony, are well-known localities. In North America they have been discovered in strata of probably the same age.[535] In fine, the genera Amblypterus and Palæoniscus may be regarded as characteristic "medals" of the geological epoch which intervened between the Devonian and Triassic formations.
[533] See Professor Sedgwick on the Magnesian Limestone. Geol. Trans. 2d ser. vol. iii.; and Prof. King’s Monograph on the Permian Fossils, published by the Palæontographical Society.
[534] Dr. Hibbert’s Memoir on the Fossils of Burdie-house.
[535] Geology of Massachusetts, by Professor E. Hitchcock.
We will next examine a few genera of the homocercal Lepidoids and Pycnodonts, whose relics are chiefly distributed in the Lias, Oolite, Purbeck, and Wealden.
Dapedius. Lign. 195.--At Lyme Regis, and other productive localities of the fossils of the Lias, large masses of angular enamelled scales, and occasionally entire specimens of the fishes to which they belonged, have for many years been collected. Sir H. De la Beche first scientifically investigated the structure of these Ichthyolites, and pointed out their characters and relations. The Dapedius (of which a restored figure is given in Lign. 195) is a flat, laterally compressed fish, with a rounded head, and fins of moderate size. The body rapidly contracts towards the root of the tail, the fin of which is large and symmetrically lobed. The mouth is furnished with several rows of small conical teeth, which are crenated at their summits, and has brush-teeth on the palatine bones; the jaws are short. The scales are rhomboidal, highly polished, and united laterally by short processes; as in many other ganoid fishes. It belongs to the Lepidoids.
The Tetragonolepis is a Liassic fish, very similar in shape to the Dapedius. It was formerly grouped with the Lepidoidei, but Sir P. Egerton has lately discovered that it essentially differs from those fishes in the character of its scales and teeth, and that it belongs to the Pycnodonts.
Lepidotus.[536] Lign. 186, 196, 197.--Scales of a dark-brown, almost black colour, with a glossy enamelled surface, and of a rhomboidal or lozenge form, and teeth equally dark and glossy, of an obtuse hemispherical figure, are very common in the Wealden strata of the south-east of England and in the Isle of Purbeck. They are called by the quarry-men fishes' scales and eyes. The collectors of the last century used to term the obtuse circular teeth of this and the related genera Bufonites, from a supposition that they were formed in the heads of toads. These relics belong to an extinct genus named Lepidotus, which contains numerous species, that are distributed in the Oolite, Purbeck, and Wealden formations. These fishes resembled the Carps in their general form, but they have no anatomical relations to that family. The body is covered with large rhomboidal scales, which are protected on the external surface by a thick plate of enamel (Lign 196, fig. 3). The lateral line, which is slightly arched, passes direct from the operculum to the middle of the insertion of the caudal fin. The head, and even the face, are cased with osseous and enamelled plates. The bones of the surface of the skull are very large, and are connected by sinuous sutures. The jaws are short and rounded, and furnished with a row of obtuse, conical, circular teeth (see Lign. 197), and several rows of sessile teeth, more or less contracted at the base, which forms a very short pedicle that is anchylosed to the bone. The fossil Lepidoti are found, for the most part, in fluviatile deposits, as in the Purbeck and Wealden strata; and it is probable they inhabited the rivers or sea-coasts, and not deep waters.
[536] Poiss. Foss. tom. ii. p. 233.
Fig. 1.--Scale, with a single process of attachment. 2.--One of the scales of the dorsal line. 3.--Scale (external surface), with a bifurcating process of attachment; the enamelled portion (which alone is visible when the scales are in their natural position on the animal) has longitudinal grooves or folds. 4.--Scale (viewed on the inner surface), having a bifurcating process of attachment and a tooth or projection on each side, to connect the scale laterally with the adjoining scales. 5.--The front ray of the dorsal fin, covered with two rows of enamelled scales, and another ray behind it.
The scales and teeth figured Lign. 196, 197, belong to the larger species of the Wealden. The remains of this fish were first collected in Tilgate Forest, and several teeth and: scales are figured Foss. Til. For. pl. v. and x.; considerable portions of connected scales have since been found; also the head entire, and the fins more or less perfect. A specimen in my collection retained a mass of the scales near the insertion of the tail, a foot wide; indicating the original to have been twelve feet long, and its body three feet broad. The scales are distinguished from other species by the folds or grooves on their enamelled surface; and the teeth by the contracted base, or pedicle, which is a little narrower than the crown (Lign. 197, and Pl. VI. fig. 10). A species (L. Fittoni) closely related to the above is equally abundant in the Weald of Sussex; the scales are not striated, and the teeth have no pedicle.
The intimate structure of the teeth of the Lepidotus is beautifully preserved, and may be easily examined in thin transverse and vertical sections, viewed by transmitted light: see Pl. VI. fig. 10. The dentine is composed of bundles of tubes, continued from the cells of the osseous base, radiating in a vertical direction to the surface of the tooth, as seen in Pl. VI. fig. 10, and giving off branches at an acute angle; but when more highly magnified, the finer branches are seen to be spread out and arched at their extremities, "presenting the appearance of the stems of corn, beaten down by heavy rain."[537]
[537] Odontography, p. 70. See the beautiful representation of this structure, pl. xxxi.
The dorsal and pectoral fins of these fishes are very strong, and consist of several bony rays. There is a double row of acuminated enamelled scales, arranged more or less obliquely, on the anterior margin of the dorsal and anal fins, and on both margins of the caudal: part of the first ray of a dorsal fin, with scales, is represented Lign. 196, fig. 5.
A small species of Lepidotus (A. minor) is common in the Purbeck limestone, and specimens may often be procured from the quarries near Swanage;[538] it has also been found at Hildesheim, in Saxony, by M. Roemer. The detached scales abound in the limestones; and the splendid fossil reptile from Swanage, figured Petrif. Lign. 38, is sprinkled with the scales and minute teeth of this fish.
[538] Fine specimens of this fish are in the British Museum (Petrifactions, p. 429), and in the Museum at Dorchester.
The majority of the species of Lepidotus belong to the Lower Oolites and the Lias. The habits of the Lepidoti, as indicated by the form and structure of the teeth, were those of fishes whose food consisted of crustaceans, shelly mollusca, &c.; for the dental organs are peculiarly adapted for the crushing and grinding of such substances; and the teeth of the adult fishes are generally worn down by use.
Pycnodus. Pl. I. fig. 3; Lign. 194, fig. 3.--The fishes of the family of Pycnodonts, so named from the thickness of their teeth, have an osseous skeleton, a flat body covered with rhomboidal scales, and flat or rounded teeth disposed in several rows on the palatine, vomerine, intermaxillary, and premandibular bones.[539] As in the Lepidotus, these teeth are constructed for crushing, and have generally a smooth, dense, convex or flattened crown, with a highly polished surface. This genus belongs chiefly to the Oolite; it is found also in the Lias, Chalk, and Wealden. A perfect fish of the genus Pycnodus (P. rhombus), from the Jura limestone, at Torre d’Orlando, near Castellamare, is figured in the frontispiece of the first volume of this work; and a vomerine bone, with teeth, from Tilgate Forest, in Lign. 194, fig. 3. In the last fossil there is a median row of flat,, elongated, transversely arched, smooth, glossy teeth, with a double alternate row of small sub-circular teeth on each side, attached to the bone, which is imbedded in Tilgate grit. Specimens of this kind, belonging to one or more species of Pycnodus, occur in the Wealden of Sussex; they were among my earliest discoveries in Tilgate Forest (Foss. Til. For. pl. xvii. figs. 26, 27). Examples are met; with in which all the teeth are shed, and the bony plate of the vomer alone remains.
[539] The intermaxillary, palatine, and vomerine bones compose the vault or roof of the mouth; the vomer occupying the middle; the intermaxillary the front; and the palatine bones the sides; the premandibular bones belong to the lower border of the mouth.
Gyrodus. Lign. 198.--In another genus of the Pycnodonts, termed Gyrodus, the crowns of the teeth are deeply furrowed, the structure of the dentine is very dense, and the pulp-cavity large and simple. One species occurs in the Speeton clay of Yorkshire, and another in the Sussex weald; but the teeth are chiefly found in the Oolite and Chalk. As in Pycnodus, the teeth are distributed in rows on the bones composing the roof, floor, and sides of the mouth.[540]
[540] An extraordinarily perfect lower jaw of a Gyrodus is to be seen in the British Museum (Foss. Brit. Mus. p. 439).
These characters are beautifully displayed in the Russian specimen, Lign. 198. This interesting fossil was presented to me by the late Stephen Cattley, jun. Esq., who collected it in 1839, in a valley near Rjeff, a village on the banks of the Volga. Mr. Cattley informed me "that many fossils are found in that and the neighbouring valleys; and the locality is frequented by Russian geologists when the season permits, which is but seldom, owing to the long duration of the snow, and the heavy rains which accompany the thaw." This specimen consists of the vomerine bone, which is of a coarse texture, and five rows of teeth; the median row consists of very large elliptical teeth; those of the lateral rows are much smaller and arranged alternately. The peculiar structure of the teeth of this genus[541] is finely displayed in this fossil. The ample, deep, and simple pulp-cavity is seen in several teeth, where the crown of dentine has been worn off, filled with a pure white calcareous spar; one of these cavities is marked a. The dentine is extremely dense, consisting of very minute calcigerous tubes, and passes into an external layer of enamel.
[541] Odontography, p. 72.
Fig. 1.--The vomerine bone of a fish, with five rows of teeth; seen from above; many of the crowns of the teeth are worn away by use, and the large pulp-cavities, filled with white spar, are exposed; as at a. 2.--Lateral view of the same. a. One of the pulp-cavities filled with spar.
[542] It is with peculiar pleasure that I inscribe this new species of Gyrodus to Sir R. I. Murchison, in commemoration of those extended and successful geological researches in the Russian empire, which have conferred additional honour on his distinguished name.
The fishes of the genus Gyrodus have the body large, flat, and elevated; the dorsal and anal fins are very long; and the tail is forked, with equal elongated lobes. The scales are laterally connected by strong processes, as in Lepidotus.
Other genera related to the foregoing occur in the Oolite; as for example, Microdon, thus named from the smallness of its very numerous flat angular teeth, arranged in many rows; Placodus, in which the teeth are few, flat, and of great size;[543] and Platysomus (flat-body), with orbicular, clavate, teeth.
[543] Odontography, pl. xliii. fig. 1, and pl. xxx. fig. 2.
In these fishes, also, the dental organs are well adapted for the comminution of shell-fish, and other hard bodies.
Cephalaspides of the Devonian System.--The remains of the three genera of Ganoid fishes that we have now to notice are of a very remarkable character, and are found exclusively in the Devonian or Old Red system; most frequently in Scotland, but also in other parts of the British Isles, and in Europe and America. These fishes agree in one general character, that of having extensive osseous plates, or scutcheons; their general aspect will be understood by reference to Lign. 199, 200, 201. There are no vestiges of the bodies of the vertebræ, which, therefore, were probably cartilaginous. These fishes constitute a distinct family with the name Cephalaspides, from the character of the first genus we propose to describe.
Cephalaspis Lyellii. Lign. 199, 200.--The most striking feature in the Ichthyolites of this genus is the enormous scutcheon, or buckler, which forms the head, and is prolonged posteriorly into two lateral horns or points; this part so closely resembles the cephalic shield of certain trilobites (see Lign. 175), that the first found specimens were supposed to be the remains of unknown crustaceans. The name Cephalaspis (buckler-head) is derived from this character. This remarkable appearance is occasioned by the intimate anchylosis of all the bones of the cranium. The body of these fishes is relatively smaller than the head; it has one dorsal fin, and terminates in a tapering tail, supporting a fin. There are two small eyes, placed towards the middle of the head. The body is covered with rhomboidal scales; and the head with discoidal scales, which are highly ornamented with radiated markings[544] (Ly. p. 344, fig. 396). There are four species of Cephalaspis at present known.
[544] Poiss. Foss. tom. ii. p. 135.
The other genera are equally unlike any recent types of the class of fishes. No perfect examples have been found, and some parts of their structure are still unknown; the annexed figures, Lign. 201, have been drawn by Mr. Dinkel (the eminent artist employed by M. Agassiz), with scrupulous accuracy, no part being introduced which is not clearly demonstrated in some one specimen; and the form of the scutcheons is made out very distinctly, to aid the collector in discriminating the detached plates, which are the most common relics of these singular beings.
Pterichthys cornutus.[545] Lign. 201, fig.1.--This fish is distinguished by its two wing-like lateral appendages; whence the name of the genus (winged-fish). These processes, like the spines on the gill-covers of the common Bullhead (Cottus gobio), are weapons of defence. In some specimens they are extended at right angles to the body (Ly. p. 345, fig. 400).
[545] The first fish of this genus was discovered by Mr. Hugh Miller, in whose charming little work, "The Old Red Sandstone; or, New Walks in an Old Field," will be found a very graphic description of the Old Red fishes; I know not a more fascinating volume on any branch of British geology. Consult also Sir P. Egerton’s elaborate paper on the Pterichthys in the Geol. Journal, vol. iv. p. 302.
The head and anterior part of the body are covered with large angular tuberculated scutcheons. The under surface of the body is flat and protected by five plates, a quadrangular plate occupying the centre. The upper part of the body is convex; the form and disposition of the dorsal scutcheons are shown in Lign. 201, fig.1. There are two eyes, which are placed in front of the lateral spines: see fig. 1. The tail is of an angular form, and as long as the body; it is covered with scales, and considered by M. Agassiz to have been the only instrument of locomotion. The British species of this genus, of which ten are known, are all very small, varying in length from one to ten inches.
Coccosteus oblongus. Lign. 201, fig. 2.--The fishes of this genus, as may be seen by the lignograph, very much resemble those of Pterichthys; in both the osseous scutcheons of the body are very similarly disposed. In Coccosteus, the head[546] is somewhat rounded; slight rounded notches on the edge of the buckler indicate the place of the eyes on the sides of the head. There is no indication of lateral spines. The tail is very long, covered with scales, and supports a fin. The plates of the body are tuberculated, as in Pterichthys. There are six or eight teeth on each half of the lower jaw (and probably as many on the upper), with a curious group of teeth situated on its symphisis. The teeth are chiefly composed of bone, passing into dentine at their surface.[547]
[546] This is of course only the cranial buckler of the animal, for, as Mr. Hugh Miller observes, "of the true internal skull there remains not a vestige. Like that of the Sturgeon, it must have been a perishable cartilaginous box."--Miller’s Foot-prints, p. 50.
[547] Miller, Rep. Brit. Assoc. 1850. Transact. Sect. p. 92. In the Annals of Natural History for November, 1848, Professor M‘Coy has given a corrected outline of the carapace, or bony cephalo-thoracic casing, of the Coccosteus. See also Miller’s Foot-prints, fig. 11.
These fishes are from a few inches to two feet in length; six species have been discovered; and their remains are the most abundant of the Ichthyolites of the Old Red. Patches of detached scales, and isolated osseous plates, are very frequent in the sandy cornstones, and the subcrystalline masses of limestone. These fragments are usually of a brilliant blue or purple colour; and, strongly contrasting with the dull red tint of the surrounding rock, are easily recognised. The colour is supposed to be due to the presence of phosphate of iron, which has communicated a similar tint to the Ichthyolites of the Caithness Schist.[548]
[548] Murch. Sil. Syst. p. 588; see also a detailed description of the Cephalaspis, Holoptychius, and other Devonian Ichthyolites, by M. Agassiz, ibid. p. 589-601.
Fossil Sauroid Fishes. (Poiss. Foss. tom. ii.)--The family of Ganoid, fishes termed Sauroid, or lizard-like, by M. Agassiz, are so named in consequence of certain peculiarities in their organization which are found in no other animals of this class, but exist in reptiles. There are but two living genera; namely, the Lepidosteus, of which there are many species that inhabit the rivers of America; and the Polypterus, that contains two species, one inhabiting the Nile, and the other the rivers of Senegal. In these fishes the bones of the skull are closely connected by sutures; the teeth are large, conical, and longitudinally striated, as in the crocodile, plesiosaurus, &c.; the spinous processes are united to the bodies of the vertebræ by suture, as in most reptiles; and the ribs are articulated to the extremities of the transverse processes; the skeleton is osseous. Even in the soft parts many analogies to reptilian structure are seen; thus the Lepidosteus has a glottis, as in the Siren; and a cellular air-bladder, with a tracheal vessel, resembling the lungs of an Ophidian (serpent). These fishes are the only living representatives of those voracious tribes of the most ancient marine fauna, whose remains abound in the Secondary formations. Their relics have often been mistaken for those of reptiles; particularly the teeth, which from their large size, conical figure, enamelled and striated surface, and internal cavity, were generally supposed to belong to crocodiles. The scales are flat, rhomboidal, and parallel to the body. The recent Lepidosteus osseus, of North America, affords a good illustration of the fossil genera; a reduced figure of this fish, from Poiss. Foss., will be found in Bd. pl. xxvii^a.; and teeth of some fossil Sauroids are represented Bd. pl. xxvii.
The teeth of the Sauroids consist of two kinds; namely, large pointed striated cones, and numerous small brush-teeth. The intricate structure of the conical teeth of the Stony-gar, or Lepidosteus, is very remarkable, and presents some analogy to that observable in the dental organs of the Labyrinthodon, an extinct genus of reptiles, of which we shall treat in a subsequent chapter. The tooth consists of a large conical pulp-cavity, surrounded by a mass of dentine, which is plicated longitudinally, its folds giving to the pulp-cavity an appearance of being divided into parallel longitudinal branches; resembling, in this respect, the base of the tooth of Ichthyosaurus, as shown in a transverse section, Pl. VI. fig 9. If we imagine these folds to be multiplied, and to have more inflections, and the pulp-cavity to be reduced in its proportions, we shall have the elegant organization of the teeth of the Labyrinthodonts (see Pl. VI. fig. 3^b). The dentine is composed of very minute calcigerous tubes, which pass off at right angles from the pulp-cavity to the periphery; and it is covered by a layer of cement, or coarser dentine, which is encased in a coat of enamel, forming the external investment of the tooth.[549] The long conical teeth are implanted in alveoli or sockets, to the walls of which they are anchylosed at their base.
[549] These remarks are based on M. Agassiz’s description and my own observations. An interesting paper, "On the Microscopic Structure of the Teeth of the Lepidostei, and their analogy with those of the Labyrinthodonts, with a plate," by Dr. Jeffries Wyman, will be found in Amer. Journ. of Science, October, 1843, p. 359.
Lepidostei.--This family, having the above described recent Lepidosteus for its type, comprises several genera, and is represented in all the formations from the Tertiary to the Coal-measures inclusive. The Lepidosteus itself has left its remains in the Eocene tertiary of England. The Lepidotus (before described, p. 604,) ranges from the Chalk to the Lias; the Pholidophorus and Dapedius (p. 603) abound in the Lias; the latter being found also in the Wealden, and the former in the Oolite and Purbeck; and the Palæoniscus (see p. 601) is a well-known Permian and Carboniferous form.
Sauroidei.--The remains of the Sauroids proper occur in the Chalk, Purbeck, Oolite, Lias, Permian, and Carboniferous rocks. The great strength and size of some of these teeth prove that the seas of those remote periods were inhabited by voracious fishes of enormous magnitude. See Petrif. pp. 432, et seq.
The teeth and jaw of a gigantic sauroid (Megalichthys), from the Carboniferous strata at Burdie-house, are figured Bd. pl. xxvii.; the sections of the teeth shown in figs. 13, 14, of that plate, illustrate the size of the pulp-cavity and the thickness of the layer of dentine. These remains were associated with the plants and crustaceans previously described; an assemblage of fossils indicating a lacustrine or estuary formation. Similar teeth have been found in the cannel-coal of Fifeshire.
In the Upper Lias of Ilminster, Somerset, the nodular calcareous rock is rich with the remains of the Pachycormus, many fine specimens of which Mr. C. Moore, of that town, has successfully worked out from their stony encasement, exhibiting the gaping, contorted fish, as it died in the convulsive throes of suffocation in the muddy sea. The little sauroid Leptolepis, of the Lias and the Purbeck, is also abundant in the above mentioned locality.
In the lithographic stone of Solenhofen, and in the Purbeck strata, Oxford Clay, and Lias of England, occur specimens of a sauroid fish remarkable for the extreme shortness of the lower jaw, and the prolongation of the upper into a beak; it is named Aspidorhynchus by Agassiz. The figure of this fossil fish is contrasted on pl. xxvii^a. Bd. with that of its recent ally, the Lepidosteus.
In the Shanklin sand and in the Galt of Kent and Sussex, large, conical, striated teeth belonging to sauroid fishes are occasionally found. They invariably occur detached, and no portion of the jaws has been observed. In the Chalk of Sussex several fine sauroid fishes have been discovered; such as the Lophiostomus,[550] Belonostomus,[551] and Caturus; the last two of which are found in the Oolite and Lias also. These have been described and figured in the late Mr. Dixon’s "Geology and Fossils of Sussex."
[550] Geol. Surv. Dec. 6. pl. x. xi.
[551] Petrif. p. 431.
The Sauroidei-dipterini[552] are found almost exclusively in the Devonian formation. Osteolepis, Diplopterus, and Dipterus are characteristic members of this family. Interesting descriptions and instructive figures of the structure of these genera are given in Mr. Hugh Miller’s late work, "Foot-prints of the Creator, or the Asterolepis of Stromness," at pp. 50 to 61.
[552] The characters of the scales of this and the next family, as well as of the Acanthodei, are succinctly given in Miller’s Foot-prints of the Creator, pp. 30, et seq.
Cœlacanthi.--This is a numerous family of sauroid fishes, that have derived their name, hollow-spine, from the central cavity in the fin-rays, which, however, may have had originally cartilaginous cores. They are found from the Devonian to the Cretaceous series. In the former, one of the most remarkable is the genus Holoptychius, distinguished by the peculiar structure of the scales; the enamelled surface of which is covered by undulated furrows. The whole body is covered by thick enamelled scales of this kind. A splendid specimen, twenty-eight inches long, and twelve wide, is figured Murch. Sil. Syst. pl. ii.^{bis.} Scales have been found exceeding three inches in length, by two and a half in width; which must have belonged to a fish of great magnitude. Ly. fig. 395.
In the Old Red Sandstone of Elgin, at a quarry at Scat-craig, some peculiar teeth occur, which possess a very remarkable structure, and have been referred to a genus of sauroid fishes, which, from the dendritical or arborescent disposition of the calcigerous tubes, Prof. Owen has named Dendrodus. These teeth are of a conical form, slightly curved, and solid throughout. On the external surface they are finely striated longitudinally, and have two opposite vertical ridges; the base is rough, and its margin rounded, as if for attachment to a shallow socket. The largest specimen is one and a half inch in length. In thin sections, viewed microscopically by transmitted light, there is a central pulp-cavity, of small size; the medullary canals pass into a few short ramifications, like the branches of a shrub, and these are distributed into irregular dilatations, simulating leaves, which resolve themselves into radiating bundles of calcigerous tubes; a portion of a transverse section[553] is shown Pl. VI. fig. 8.
[553] Odontography, pl. lxii. B fig. 2; and Cycl. Anat. Art. Teeth.
[Sidenote: ASTEROLEPIS.]
In Mr. H. Miller’s charming work just referred to we have a full and clear account of the singular fossil fish the Asterolepis. Remains of this gigantic Ganoid were first found in Russia.[554] Its name is derived from the stellate markings on the dermal plates of the head, which are of great size, and form a strong expanded buckler, the orbits of the eyes being situated near the anterior border. (See Miller’s Asterolepis, pp. 74, et seq. figs. 27-29.) Lign. 202 shows a part of the lower jaw of this fish, seen on the inside. Along the upper margin are seen a "thickly set row of small broadly-based teeth,"--these are ordinary fish-teeth; and behind this edge-row of small teeth, b, there occur "a thinly set row of huge reptile-teeth, based on an interior platform of bone, which formed the top of the cartilage enclosing box composing the jaw." (Miller.) These large teeth, a a, are longitudinally striated, and generally bear two sharp lateral cutting edges.[555]
[554] Casts of the fossil bones from Russia are in the British Museum; Petrif. p. 435.
[555] The microscopic structure of both kinds of teeth is elegantly figured in Mr. Miller’s work, pp. 81, 82.
Macropoma Mantelli. Wond. p. 348. (Foss. South D. tab. xxxvii. xxxviii.; Petrif. p. 436, Lign. 89.)--This Cœlacanth fish is from one to two feet in length, of an elongated fusiform shape, with a large head and two dorsal fins; the anterior fin is imbedded, and has seven or eight strong spinous rays, the first two of which have numerous spines. The opercula are very long and large (hence its name); the scales[556] are garnished with adpressed spines, disposed in semicircular rows (see Lign. 185, fig. 2). The teeth are small, conical, and numerous.
[556] See Prof. Williamson’s Memoir, Philos. Trans. 1849, p. 435; pl. xliii. figs. 27, 28.
Several specimens of this fish are almost perfect. The bones of the cranium, the jaws, teeth, opercula, branchial rays, palatine arches, the surface of the body covered with scales, all the fins, the pelvic bones, the vertebræ and their apophyses, all remain.
In one example, the vomer, covered with minute teeth, is exposed. But the most extraordinary fact relating to these Ichthyolites, is the preservation, in every specimen, of the air-bladder;[557] even its membranes remain, and separate in flakes; and the ramification of the minute vessels is visible under a high magnifying power. In some instances this bladder is displaced and much distended; but in general it occupies its natural position, and retains its elongated, sub-cylindrical form, with a few annular constrictions or folds.
[557] See Williamson, op. cit. pp. 462-165, and figs. 29, 30.
[Sidenote: COPROLITES. COLOLITES.]
Coprolites. Lign. 139, figs. 1, 2.--In more than one example the solid earthy residue of digestion, in small lumps or pellets, of a conical form and spirally convoluted, lies in the abdomen of these fossil fishes. Fossil excrementitious substances of this kind are termed Coprolites (Bd. p. 198, pl. xv.); they occur in many deposits, and belong not only to fishes, but to large reptiles and other animals. Lign. 139, p. 432, fig. 1, represents the coprolite of a Macropoma; and fig. 2, that of a species of Shark, from Hamsey. The convoluted appearance of these bodies arises from the peculiar organization of the intestinal canal of the original fishes, in which, as in the recent Dog-fish, a portion of the intestine was spirally twisted, the tube forming several gyrations; and the passage of the calcareous substance through this constricted canal gave rise to the structure observable in the coprolites. In the Macropoma the gyrations appear to have been few; seldom more than five or six turns being apparent. In the fossil Sharks the convolutions are more numerous, ten or twelve occurring in the length of an inch. In many of the coprolites, the impression of the mucous or lining membrane of the intestinal canal may be detected. Some of the coprolites of the Macropoma are not convoluted; probably from having occupied the upper part of the digestive tube, and therefore not having been moulded in the spiral part. (See Geol. S. E. p. 145. Foss. South D. tab. ix.) Minute scales and bones of fishes are occasionally imbedded in the substance of the coprolites, affording evidence of the carnivorous habits of the Macropoma.
Cololites. Bd. pl. xv^a--It will be convenient to notice in this place those curious fossils which occur in detached masses in the limestone of Solenhofen, and used to be known to collectors by the term lumbricaria, from the supposition that they were petrified earth-worms. An excellent representation of a fine specimen is given by Dr. Buckland in the plate referred to above. These convoluted bodies M. Agassiz, with his wonted sagacity, has ascertained to be the intestines of fishes; and has therefore named them Cololites. Although generally found isolated, specimens occur in which they are imbedded, like the coprolites of the Macropoma, in the abdominal region of fishes. The frequent occurrence of the Cololites apart from the body, is explained by the usual process of decomposition in fishes. M. Agassiz remarks that dead fish always float on the surface of the water with the belly uppermost, until the abdomen bursts from distention. The small intestines are then expelled by the evolved gases through the aperture, and soon become detached from the body. I have observed indistinct traces of similar remains in the beds of chalk in which fossil fishes most prevail. Dr. Buckland mentions the discovery, by Lord Greenock, of a mass of petrified intestines distended with coprolite, and surrounded by the scales of a fish, in a block of coal-shale from the neighbourhood of Edinburgh (Bd. p. 199).
Dercetis elongatus. Wond. p. 349.--Before proceeding to the investigation of examples of the next order, I will describe a highly interesting Ichthyolite, which in a mutilated state is extremely common in some of the chalk strata of the South-east of England; it is noticed in Foss. South D. p. 232. This fish is placed by M. Agassiz in his family of ganoidians, termed Scleroderms; and he mentions that another species has been found in the chalk of Westphalia. The Dercetis has a very elongated body, with a short head terminating in a pointed beak; the upper jaw is a little longer than the lower, and both jaws are armed with long, conical, elevated teeth, and several rows of very small ones. On each side of the fish there are three rows of osseous scutcheons like those of the Sturgeon (see Wond. p. 349); the body was also covered with numerous small scales. From the form of the body somewhat resembling that of the eel, being very long, and sub-cylindrical in uncompressed examples, the specimens are generally called "petrified eels" by the quarry-men. The examples usually found consist of the elongated body, more or less compressed, and irregularly covered with patches of scales confusedly intermingled; among which traces of the scutcheons may sometimes be distinguished. These specimens occasionally exceed two feet in length, by one or two inches in breadth; with neither extremity perfect, and without any vestige of the fins.[558] The example figured Wond. Lign. 74, is the only instance in which I have seen the cranium perfect. The scutcheons in the Westphalian species have a prominent longitudinal ridge or keel, and their surface finely granulated; they are so large that the whole body of the fish is covered with them.
[558] See Foss. South D. pl. xl. fig. 2, and pl. xxxiv. figs. 10 and 11.
[Sidenote: CTENOID FISHES.]
Fossil Ctenoid Fishes (Poiss. Foss. tom. iv.).--The fishes of this order have imbricated laminated scales, the posterior margins of which are round and finely pectinated; i. e. divided into little teeth, like a comb. These scales are nearly circular, but more or less elongated; and, as the laminæ of which they are composed successively diminish from the lowermost to the uppermost, the pectinated margin of each being apparent, the surface is very scabrous; the front edge is sinuous. The common Perch is the type of the Ctenoidians. The teeth of these fishes are invariably small, and either villous or brush-like.
From the numerous fossil genera I select, in illustration of the characters of this order, the Beryx; of which four species occur in the English Chalk, and three others in the Chalk of Bohemia and Westphalia. Of this genus, which is closely related to the Perch (Perca), two living species inhabit the seas of Australia. The bones of the skull have dentated crests; the dorsal fin has spinous rays in front, which are united to the soft rays; the margin of the caudal fin has little spinous rays.
The Beryx is one of the most ancient representatives of the Perch tribe, and of the Ctenoid order.
Beryx Lewesiensis.[559] Wond. p. 351; Petrif. Lign. 90.--This is one of the most common of the Ichthyolites of the Chalk of the South-east of England; it is called "Johnny Dory" by the quarry-men; the specimens are from six to twelve inches long. It occurs also in the Chalk of Westphalia. The outline of the perfect form of this species, Wond. p. 351, by Mr. Dinkel, conveys an accurate idea of its external characters. It has one dorsal fin, with several spinous rays in front of the soft ray. The head is very large, and the opercular pieces are ornamented with sculptured rays; the margins of the jaws are covered with a broad band of brush-teeth. The orbit is large, and often contains the capsule (sclerotica) of the eye. The rays of the gills are short and thick, five are preserved in some examples. The scales are very large; about twenty-five in the median row; their posterior margins have several concentric rows of spines (see Lign. 185, fig. 3, p. 567). The lateral line is often distinctly apparent, in the form of a tube, contracted behind and expanded in the centre of the scale. The vertebral column is composed of large short vertebræ, with very long apophyses; the ribs are slight.
[559] The fossil discovered by me, and figured in Foss. South D. tab. xxxvi. was the first perfect fish obtained from the Chalk. This fish was first described by me, Foss. South D. tab. xxxv. xxxvi. as Zeus Lewesiensis; M. Agassiz has very properly referred it to the genus Beryx; but he has also substituted another specific name; which is wholly unwarrantable, for that first imposed ought to be retained; see p. 518, note.
Beryx superbus.--This is a larger species, sometimes thirteen inches long, with very large and broad scales. It has been found in the lower chalk at Lewes. See Dixon’s Foss. Sussex, tab. xxxvi. fig. 5.
Beryx radians. Wond. p. 350.--This species is smaller, and relatively longer, than B. Lewesiensis; it generally occurs in the Chalk-marl, and is invariably of a very dark colour, the scales having a polished or glossy aspect. The scales are small, with a simple row of diverging spines on the posterior edge. The scales of the lateral line are peculiar; the mucous canal is not formed of a series of simple tubular cylinders, as in B. Lewesiensis, but is divided into several branches, as may be seen with a lens of moderate power. There are more than thirty scales in the length of the lateral line.
Beryx microcephalus (Poiss. Foss. tom. iv. tab. iv^c.; and Dixon, Foss. Sussex, tab. xxiv. fig. 3).--This fish is distinguished by its slender form, and the extreme smallness of the head, as the name implies. The scales have one row of very thick spines on the posterior margin; they are more elevated, and shorter than in B. radians; those of the lateral line are pierced by an elongated conical tube, and are not ramified as in the last species. This ichthyolite, like the B. radians, occurs in the Chalk-marl, and in a similar state of mineralization.
Smerdis minutus. Lign. 203.--A pretty ctenoidian fish, from one to three inches long, about the dimensions of a perch a year old, is very common in the marls of Aix in Provence, Wond. p. 260; and many are often found grouped together in every variety of position. This species is characterised by the elevated anterior rays of the dorsal, and the wide and very forked caudal fin.
Several ctenoidian and cycloidian fishes have been found in the north of the Brazils, by Mr. Gardiner, in strata probably of the Cretaceous epoch.
Fossil Cycloid Fishes. (Poiss. Foss. tom. v.)--This order comprises the fishes possessing scales of a cycloid, or circular, form, with smooth margins, and composed of plates of horn or bone, without enamel. It contains numerous families, including the Scaroids, or Parrot-fishes, and the Scomberoids, or Mackerel tribe, which are Acanthopterygians, the Lucioids, or Pikes, Clupeoids, or Herrings, Salmonoids, or Salmon tribe, and the Cyprinoids, or Carps, which are Malacopterygians, as well as other families. The fossil remains of this order are exceedingly numerous, particularly in the Tertiary and upper Secondary deposits. A genus of Salmonidæ, discovered in the White Chalk of Sussex, will serve to exemplify the characters of the fossil cycloidian fishes.
Osmeroides. Plate II. and Wond. p. 344, 347.--Two species of this genus occur in the Chalk, near Lewes; and principally, if not exclusively, in the Lower Chalk, without flints. They are exceedingly beautiful Ichthyolites, and are almost invariably found with the body but little compressed; the fish, in many examples, is as round and perfect as when living. The entire cranium, the opercula and branchial rays, and all the fins are preserved in some examples. These fishes belong to the Salmon family,[560] and are nearly related to the Smelt (Osmerus); whence the name of the genus. There are two species, easily distinguishable. The first (O. Mantelli, Pl. II.) has a short, sub-cylindrical body, and seldom exceeds eight or nine inches in length; the other (O. Lewesiensis) has an elongated and elliptical body, and sometimes attains a length of fourteen inches. The dorsal fin too in this species has more rays than in the other. The fossil figured in Pl. II. is a very remarkable specimen of the first species. It is nine inches in length; and the chalk has been cleared away, so as to expose the entire fish, six inches in relief above the surface of the block, Petrif. Lign. 92, p. 445. The fish is lying on its back, with the mouth open, and the opercula, or gill-covers, and the branchial arches expanded; the pectoral and ventral fins, and the dorsal fin, are in their natural position; the five rays of the dorsal are erect; of the caudal fin, or tail, but slight indications remain. There is but one dorsal fin; but in a specimen of O. Lewesiensis there is a trace of the little adipose process observable between the dorsal fin and the tail, as in the recent species of Salmonidæ. A magnified view of one of the scales is represented Lign. 185, fig. 4, p. 567.
[560] They were first described by me in Foss. South D. p. 235, tab. xxxiii. and xl. as Salmo Lewesiensis.
Of the Cyprinoids, or fishes of the Carp family (Malacopterygians), the recent species of which are inhabitants either of fresh-water, or the brackish waters of the mouths of rivers, many fossil species occur in the fluviatile and lacustrine deposits of the Tertiary formations. In their character of omnivorous fishes, the Carps then, as now, formed the principal mass of the finny population of the lakes, and in their turn served as food to the carnivorous tribes, as the pikes, eels, &c. Several species are found in a beautiful state in the schists of Œningen, and in the Tertiary marls at Aix. Many of the layers of marl at the latter locality are covered with groups of fishes of the family Cyprinodonts, the recent species of which are of a small size, and inhabit the fresh-water lakes of temperate zones. Lign. 184, p. 562, represents a portion of a large slab of marl in the cabinet of Sir R. Murchison, which is covered with scores of a species resembling a recent fish (Lebias) in the profile of its head, and the form of its fins. It is named Lebias cephalotes, from the relative largeness of its head. The black appearance of the abdomen in many of these Ichthyolites indicates the original situation of the intestines and of the liver, which is largely developed in the fishes of this family, and contains much colouring matter.
Saurocephalus and Saurodon. Lign. 204.--In the same quarry, near Lewes, from which the first entire fish of the Sussex Chalk was obtained, teeth of a very peculiar character were, many years since, occasionally discovered.[561] These teeth are of a lanceolate form, much compressed; with entire, sharp edges, terminating in a point; the fang is single, and broad; the surface of the crown is glossy, and marked with fissures filled with chalk (see Lign. 204, fig. 1). Teeth of this kind, attached to portions of the jaw, were subsequently found in the Chalk at Brighton and Lewes. Similar remains were collected from the Cretaceous marls of Missouri and New Jersey, in the United States. The American specimens comprised two closely allied genera, which, from the supposition that the fossils were the relics of reptiles, were respectively designated Saurocephalus and Saurodon.[562] Examples of the teeth and jaws of both genera have been discovered in the Sussex Chalk (see Lign. 204).
[561] Foss. South D. tab. xxxiii. p. 228.
[562] American Phil. Trans, vol. iii. new series, pl. xvi. on the Saurodon, by Dr. Hays; and Journal Acad. Sciences, Philadelphia, vol. iii. on the Saurocephalus, by Dr. Harlan.
Fig. 1.--Tooth of Saurocephalus lanciformis. Lewes. 2.--Teeth of Saurocephalus striatus. Brighton. Portion of the jaw, with five teeth. 3.--Fragment of a jaw, with two perfect teeth, and the base of another, of Saurodon Leanus. Kemptown, Brighton. 4.--Enchodus halocyon. Left branch of the lower jaw, with teeth; and one front tooth of the opposite portion. Lewes.
M. Agassiz retains the names imposed by the American naturalists, and has placed these genera in the family of Scomberoids (Mackerel, Swordfish). They belong to the Pharyngognathi of Muller. The teeth are disposed in a single row, and fixed in deep sockets by a simple root, or fang, which is frequently somewhat excavated by the pressure of a successional tooth. In Saurodon Leanus the crown of the tooth is angular, and barbed, and supported on a sub-cylindrical shank, or stem (see Lign. 204, fig. 3). The microscopical structure of these teeth presents that peculiar reticulated disposition of the medullary canals throughout the entire body of the tooth, which is only found in the dental organs of fishes.
Hypsodon Lewesiensis. (Foss. South D. tab. xlii.)--The Sussex and Kentish Chalk also contain the remains of a very large fish, belonging to the Pharyngognathi, with extremely upright, long, conical, compressed, pointed teeth, which, like those of the Saurodon, are implanted in sockets. These teeth are commonly of a delicate fawn colour externally and of a dark brown internally; having a large simple pulp-cavity. In Foss. South D. (tab. xlii.) are represented portions of an intermaxillary and jaw-bone with teeth; a vertebra, deeply biconcave; and a large bone, apparently a branch of the os hyoides; all found in the same block of chalk.[563]
[563] A magnificent specimen (now in the British Museum) displays, on the same slab of chalk, a large portion of the cranium, teeth, several vertebræ, ribs, and many other bones, belonging to a fish of considerable magnitude. Petrif. p. 444.
Enchodus (sword-tooth) halocyon. Lign. 204, fig. 4. (Poiss. Foss. tom. v. tab. xxv^c.)--The specimen figured is a portion of the lower jaw, with one row of elongated, conical, slightly curved, pointed teeth; the two anterior teeth being much longer and larger than the others; it affords a good illustration of the dental organs of Enchodus; a genus of Acanthopterygian fishes, the jaws and teeth of which are often found in the Sussex Chalk. The teeth are of various sizes, and attached by anchylosis, one row on the premandibular bone, and another irregular row of smaller teeth to the inside of the lower jaw. The two anterior teeth are very large, and of a peculiar form: their base is wide and solid, and the shank of the tooth is suddenly contracted immediately above, and becomes elongated into a point. These teeth are generally of a dark colour, have a glossy aspect, and are very brittle; differing so remarkably in this respect from the shark’s teeth, with which they are usually collocated, that mere fragments can be readily identified. The external surface of the lower jaw is marked with finely granulated, longitudinal ridges or striæ.[564]
[564] A fine example of the lower jaw, with twelve teeth, is figured Foss. South D. tab. xli. and another, with the upper jaw and teeth, Geol. S. E. p. 140. Beautiful figures of the remains of Saurocephalus, Enchodus, Hypsodon, and many other fine Chalk fishes, are given in Dixon’s Fossils of Sussex, 4to. 1850.
Mr. Toulmin Smith, of Highgate, has in his Museum a portion of the lower jaw with fifteen teeth of a small individual, imbedded in the centre of a flint nodule, from Gravesend, which was discovered by accidentally breaking the stone. The bone of the jaw and the teeth are of a dead white colour, and appear not to be in the slightest degree silicified; but in those teeth which are broken the pulp-cavity is filled with quartz, which must have transuded through the walls of the teeth.
Ichthyolites of recent species.--The distinguished naturalist to whose labours in fossil Ichthyology we have been so largely indebted, states, that of the many hundred species submitted to his notice, but one can be identified with any fish now living. This conclusion must, however, be received with some reservation; for, among the fossil genera, founded on the teeth, there are species which certainly cannot be distinguished from recent forms. And in the diluvial drift at Breslau, associated with the bones of the fossil elephant (Elephas primigenius), the remains of a pike, closely resembling the common European species, have lately been discovered.[565]
[565] Agassiz, Poiss. Foss. tom. v. p. 68.
The exception above alluded to, is a little Malacopterygian fish, rather larger than a Sprat, called the Capelan (Mallotus villosus), which inhabits the banks of Newfoundland, and other parts of the coasts of the northern seas. Fossil specimens of this fish.[566] occur in nodules of indurated marl or clay, on the coast of Greenland.[567] It is supposed that these Ichthyolites are of very recent date: and that similar fossils are in the progress of formation.
[566] See Poiss. Foss. tom. v. pl. lx., in which the skeleton of the recent fish, and specimens of the fossil species, are represented.
[567] Similar fossils have been obtained from the "Drift" on the Saco River, thirty miles north of Portsmouth, New Hampshire. See Lyell’s Second Visit to the United States, vol. i. p. 29.
Ichthyopatolites, or imprints of the pectoral fin-rays of certain fishes. Under this name Dr. Buckland described certain problematical markings observed on a flag-stone from a coal-pit at Mostyn, in Flintshire, and now in the Geological Society’s Museum. It consists of curvilinear scratches or imprints, disposed symmetrically at regular intervals on each side a smooth level space, about two inches wide, which may correspond to the body of a fish, the pectoral fins of which Dr. Buckland suggests were the instruments by which the markings in question were formed.
These scratches follow each other in nearly equidistant rows of three in a row, and at intervals of about two inches from the point of each individual scratch to the points of those next succeeding and preceding it; they are slightly convex outwards, three on each side the median space, or supposed track of the body of the fish. Dr. Buckland, in the memoir referred to, shows that these markings cannot be referable to the imprint of the feet or claws of reptiles, and points out the structure of the bony anterior rays of the pectoral fins, as in certain Siluroid and Lophoid fishes, and in the Climbing Perch (Anabas scandens), or the Hassar (Doras costata), and refers also to the ambulatory movements of the common Gurnard, in corroboration of this opinion.[568]
[568] Proceedings of Geol. Society, vol. iv. p. 204.
* * * * *
Geological Distribution of Fossil Fishes.--From the incidental notices of the geological habitats of the fossil fishes enumerated in our survey of this class of beings, the reader cannot fail to have remarked, that the most recent strata abounded in forms related to the inhabitants of the existing seas and rivers; while the most ancient teemed with species and genera of families altogether extinct, or of prodigious rarity in the recent fauna.
In general terms, it may be stated, upon the authority of M. Agassiz, that the Ichthyolites of the Tertiary deposits approach in their characters to the living genera, but all the species are extinct. The newer Tertiary, as the Crag, contain genera common to tropical seas, as the large sharks (Carcharias), and eagle-rays (Myliobates), &c. In the Eocene, or most ancient Tertiary, as the London and Paris basins, Monte Bolca, &c., many of the Ichthyolites are closely related to recent genera. Of the Chalk fishes, a few only are of recent genera, but the majority are still allied to Tertiary forms. In the Chalk, the Pharyngognathi, Acanthopteri, and Malacopteri are met with as new types; and indications of the Hybodontidæ, Sauroidei, and Cœlacanthi (the last derived from the Devonian, and the other two from the Carboniferous Limestone) appear for the last time.
The ichthyic fauna of the Cretaceous deposits is closely related by the majority of its family groups with that of the series of strata from the Lias to the Wealden, inclusive. In and above the Lias all the ganoid fishes are homocercal. Below the Lias, the genera and species are far more removed from existing types, and almost all are heterocercal.
Of the eight thousand living fishes known to naturalists, three-fourths belong to the Cycloid and Ctenoid orders, and of these no species are known below the Chalk; the other fourth is referable to the Placoids and Ganoids, of which there are comparatively but few existing species. Yet fishes of these two orders almost solely flourished during the ancient Secondary formations; for below the Lias, the predominant recent orders are altogether absent. Beneath the Coal, true carnivorous fishes, with trenchant teeth, are almost unknown; but omnivorous species, with either brush or obtusely conical teeth, and great sauroid fishes, are the prevailing representatives of the class.[569] In fine, the Ichthyolites of the different formations constitute two grand groups, which have their boundary line at the base of the Cretaceous deposits. The first and most ancient comprises the Ganoids and Placoids; the second, more intimately related to existing types, comprehends forms more diversified; these are principally Ctenoid and Cycloid, with a small number of the two preceding orders, which insensibly disappear; and their few living analogues are very distinct from the ancient species. Now, although deductions of this nature may require to be modified with the progress of knowledge, yet the generalizations thus obtained are founded on so vast an accumulation of facts and observations, as to render it improbable that they will be materially invalidated by future discoveries; for they remarkably accord with the results derived from the investigation of the fossil remains of all the other classes of animals. The most modern deposits contain the remains of animals allied to the existing species; the most ancient, of forms altogether extinct, or of excessive rarity in the recent faunas. The discovery of existing species, or genera, in the most ancient strata, would modify, but not destroy, the inferences deduced from the facts hitherto obtained; and every geologist is prepared to find that such may be the case.
[569] In the several chapters on the different formations, as arranged in the Wonders, the student will find succinct notices of the distribution of the genera of fishes throughout the fossiliferous deposits. A list of the Chalk species known in 1848 is given at pp. 356-359, Wond.
Thus of the Sharks, with triangular notched teeth, which are so common in the Tertiary formations, and were formerly unknown in the ancient Secondary, one representative has been found in the Carboniferous system (see p. 595). But, if teeth of this type should hereafter be discovered in every Secondary deposit, the great preponderance of these fishes over the Sauroid in the Tertiary, and in the existing seas, would not be the less remarkable.
* * * * *
On Collecting and Developing Fossil Fishes.--From what has been advanced, the reader will have obtained a general knowledge of the fossil remains of this class that are likely to be met with in particular deposits. Thus, he will expect to find the teeth of large sharks and rays in the Tertiary clays and sands; and skeletons and perfect specimens of numerous Ctenoid and Cycloid fishes in the laminated marls and fine limestones of the same formations. In the Chalk, with numerous teeth of sharks, he may discover splendid examples of Cycloid and Ctenoid fishes; and, in the Wealden, large Ganoidian forms. Passing to the ancient Secondary strata, the extraordinary buckler-headed and Sauroid fishes will arrest his attention; and their vestiges will be found, more or less perfect, in the shales and limestones, and in the indurated nodules of clay and sandstone.
The detached teeth of fishes in Tertiary sands and clays may be easily obtained entire, and should be arranged in the same manner as the shells (see p. 442), either in trays, or on boards. The triangular teeth, with lateral denticles, must be carefully extracted, so as to preserve those appendages on which the specific and generic distinctions of many Ichthyolites depend. M. Agassiz particularly recommends the preservation of all the specimens collected together in the same locality, as many may probably belong to the same individual, and thus the dental organization of the original be determined. Teeth collected from the same stratum in different places, should not, therefore, be mixed together. Several series of the same kind of teeth should be preserved, and as many as possible of each kind; for specimens apparently identical may prove to be highly instructive as a series. I have often had occasion to regret the disposal of supposed duplicates, in my earlier researches, which would have tended to elucidate the characters of those specimens which were retained.
The Ichthyolites, and their detached teeth and fins, in the Chalk and other soft limestones, may be cleared by means of a penknife or graver and small sharp chisels. It is preferable to leave the teeth attached to small blocks of the chalk; as in the examples, figured Lign. 193. But to develop the beautiful Chalk Ichthyolites, particularly those of the Osmeroides, Macropoma, &c. some practice and considerable dexterity are required. The compressed fishes, as the Beryx, like those in the Tertiary limestones, often lie in the sedimentary plane of the stone, and may be sufficiently exposed, by a blow of a hammer or a pick, to show the nature of the fossil, and admit of being easily developed. But the fishes with sub-cylindrical bodies very commonly split asunder in a transverse direction: and those with spinous scales, as the Macropoma, adhere so firmly to the chalk, that, to display the external surface of their scales, the surrounding stone must be removed piecemeal, in the manner described for the Chalk crustaceans (see p. 544). The collector who sees the splendid Chalk fishes in the British Museum,[570] and learns that they were found in the Chalk of Kent and Sussex, will be grievously disappointed, upon visiting the quarries from which they were obtained, if he expects to discover specimens with any considerable portion of the scales, or body, exposed. It was many years before the quarry-men acquired the tact they now possess, of detecting, from very slight evidence, the presence of an Ichthyolite in a block of chalk: patches of scales, which the quarry-men called "bran," and detached sharks’ teeth, "birds beaks," and "snakes' tongues," and teeth of Ptychodus, "slugs," being the only remains of fishes generally observed and laid aside by the workmen.
[570] Petrifactions, pp. 441, 444.
The fossil Salmon or Smelt (see p. 626), which may be considered as one of the most extraordinary of the Chalk fishes found in England, affords an excellent illustration of the mode of developing the Ichthyolites of this formation. This interesting fossil is delineated on a small scale, in three different states, in Plate II.; and affords a good practical lesson for the young collector. Among some blocks of chalk which a recent fall in one of the quarries near Lewes had brought to light, was a large mass split asunder, and exposing on each corresponding surface an irregular oval marking of a yellowish brown colour; this appearance is represented _Pl. II. fig. 1_. Presuming that these markings were produced by a transverse section of the body of a fish, the two blocks were trimmed into a portable size, and accurately cemented together with very hot, thin, fresh glue. When consolidated, some of the chalk was chiselled off in the supposed longitudinal direction of the enclosed fish, and part of the body, covered with scales, was exposed, as Pl. II. fig. 2. With the view of ascertaining the extent of the Ichthyolite, some of the surrounding stone was then removed towards each extremity of the block, and traces of the fish were discovered, as shown in the same figure. The task of completely developing the fossil was thus rendered comparatively easy; the chalk was chiselled, cut, and scraped away, till the perfect fish, as seen in fig. 3, was developed.[571] The block was then reduced to a convenient size, and the edges sawn smooth. The chalk is easily cut with a carpenter’s saw; the instrument should be short and strong, and the teeth of moderate size.
[571] The figure in Pl. II. is too small to convey an accurate idea of this Ichthyolite, which is now in the British Museum; see Petrifactions, pp. 445, 446. M. Agassiz’s figure very inaccurately represents the original. A beautiful lithograph of this fish, by Mr. Pollard, of Brighton, was published in the Catalogue of the Mantellian Museum, 1836.
When a portion of the body of an Ichthyolite of this kind is found in a block of chalk, and the fracture of the block appears to be recent, diligent search should be made for the corresponding piece; for it may probably be found to contain the other part of the fish. A splendid specimen of Osmeroides Lewesiensis, more than a foot long, was thus obtained. The quarry-men, in a block of chalk which a recent fall had thrown down, discovered a few inches of the caudal portion of the body of a fish; on the broken surface of the stone, a section of the body was distinctly seen, as in the specimen previously described. Search was made among the fallen masses for the corresponding piece, but without success. Upon observing the face of the quarry exposed by the recent fall, on a projecting block, many yards above our reach, a discoloured spot was indistinctly seen, and it was conjectured that this might prove to be the other moiety of the Ichthyolite. The workmen were directed to preserve this block if possible; but it remained in situ several months, and until the rock was again blasted; when the stone so long coveted rolled away from the fallen mass, and fortunately was soon discovered. It proved to be the corresponding portion of the fish; with the head, opercula, branchial arches, pectoral fins, and the anterior part of the body covered with beautiful cycloid scales. In the preparation of fossils of this kind, glue as the cement, and a paste made of plaster of Paris with thin glue, to fill up the crevices and strengthen the block, are the materials I have employed. The fossil remains of fishes in other rocks require to be extracted and developed in the manner previously directed for the Echinoderms, Cephalopoda, &c. (pp. 332, 497.)
The collector may be reminded, that Otolithes, or ear-stones (p. 574), are found in the Crag of Norfolk, and other Tertiary strata; and that Coprolites, associated with minute scales, bones, &c. of small fishes, constitute, in some localities, layers of considerable thickness and of great extent. The "bone-bed" of the Lias, near Westbury, and that of the Ludlow series on the banks of the Teme, near Ludlow,[572] are well-known examples of such a deposit.
[572] See Mr. Strickland’s interesting notice of the distribution and contents of this "bone-bed," in the Quart. Geol. Journ. vol. ix. p. 8
Microscopical Examination.--A few words on the microscopical examination of the remains of fishes may be useful. The structure of the large, and the forms of the minute scales, may be seen by a common lens, and without preparing the specimens. But for the examination of the intimate organization of scales, teeth, &c. the microscope is required; and the method directed for the investigation of flint (p. 373) should be employed. The scales, portions of the membranes of the air-bladder, stomach, &c. and thin chips of the teeth, rendered temporarily transparent by oil of turpentine, or permanently so by Canada balsam, should be viewed by transmitted light. But the intricate structure of the dental organs, the medullary canals, and the calcigerous tubes, cannot be successfully investigated without the aid of the lapidary, or the adoption of the process described at page 67 for the preparation of fossil wood for microscopical examination.
BRITISH LOCALITIES OF FOSSIL FISHES.
⁂ The detached teeth, scales, vertebræ, &c. of fishes are so extensively distributed, that there is scarcely a cliff or quarry of fossiliferous rock in Great Britain, that does not contain some examples. The following list of localities must, therefore, be regarded as merely directing the student to a few places, in which particular fossils of this class have been discovered.
Abergavenny. Mt. L. Teeth of Psammodus, Orodus, &c.
Armagh, Ireland. Mt. L. Numerous teeth and spines.
Arundel, Sussex. Cret. Quarries in the neighbourhood; beautiful Chalk fishes.
Aust Cliff, near Westbury, Somersetshire. Lower Lias. Pholidophorus, &c. Base of Lias. In a layer called the bone-bed, containing bones, scales, teeth, and Coprolites of fishes. Teeth of Ceratodus, &c.
Axmouth. Base of Lias: Bone-bed. Numerous scales, bones, and teeth. Saurichthys, &c.
Barrow-on-Soar. Lias. Dapedius.
Bracklesham Bay, Sussex. Eocene. Magnificent specimens of Rays, as Myliobatis, Aëtobatis, and of Chimæroids were collected by the late F. Dixon, Esq., and are now in the British Museum.
Brighton. Cret. Chalk quarries in the vicinity. Beryx, Dercetis, Saurocephalus, Saurodon, and the common species of teeth, &c.
Bristol. Mt. L. The usual species of Psammodus, Orodus, Onchus, &c.
Burdie-house, near Edinburgh. Carb. Palæoniscus, Megalichthys, Holoptychius, &c.
Caithness, Scotland. Old Red. Dipterus, &c.
Charing, Kent. Many fishes in the Chalk.
Chatham, Kent. Cret. Beryx, Hypsodon, and the usual teeth, &c.
Cheltenham. Base of Lias. In the bone-bed teeth, scales, Coprolites.
Clayton, Sussex. Lower Chalk. Beryx microcephalus, and other rare Ichthyolites.
Clifton, near Bristol. Mt. L. Psammodus, Orodus, &c.
Cromarty, Scotland. Old Red. Coccosteus, Pterichthys, &c.
Cuckfield, Sussex. Wealden. Lepidotus, Hybodus, Acrodus.
Cullercoats, Durham. Permian. Palæoniscus, &c.
Dinton, Vale of Wardour. Purbeck. Leptolepis, Ceramurus, &c.
Downton Hall, near Ludlow. Devonian. Cephalaspis, Dipterus, &c. U. Sil. In a quarry on the banks of the Teme, a fish-bed composed of scales, teeth, and Coprolites, in Upper Ludlow limestone.
Dudley. Sil. Ichthyodorulites.
Dungannon, Ireland. Permian. Quarry at Rhone-hill; numerous small Palæonisci, P. catopterus.
East Thickley, Durham. Magnesian Limestone. Palæonisci.
Glammis, Forfarshire. Devon. Cephalaspis, Gyrolepis, Dipterus.
Gravesend and Northfleet. Chalk-pits rich in fish-teeth, &c.
Hastings. Wealden. Lepidotus, Hybodus.
Ilminster, Somerset. Upper Lias. Pachycormus and Leptolepis.
Leeds, Middleton Quarry. Carb. Layers of fish-coal, abundance of remains of Megalichthys, Holoptychius, &c. (Geol. Proc. iii. p. 153.)
Lewes, Sussex. Cret. All the fishes of the British Chalk. See Wond. pp. 356-359.
Lyme Regis. Lias. Dapedius, Hybodus, Squaloraia; and numerous other species and genera.
Newhaven, near Leith. Carb. On the shore, nodules of ironstone with fishes and Coprolites. Amblypterus, Palæoniscus.
Sheppey, Isle of. Tert. Numerous teeth of Rays, Sharks, &c., and other Ichthyolites in great abundance.
Shotover, near Oxford. Kimmeridge Clay. Ischyodus, Hybodus, &c.
Southend, Essex. Eocene. Fish-bones and teeth (Pisodus, &c.) are found on the shore along the foot of the cliff.
Speeton, Yorkshire. Galt. Macropoma Egertoni; and many other fishes.
Steyning, Sussex. Cret. In the marl-pits, Coprolites and teeth of Sharks are abundant.
Stonesfield. Great Oolite. Hybodus, Lepidotus, Leptacanthus, &c.
Swanage. Purbeck. Lepidotus, Hybodus, Ophiopsis, &c.
Thurso, Scotland. Devonian. Asterolepis, &c.
Westbury, near Bristol. Base of Lias. Bone-bed with numerous remains.
Worthing. Cret. Beautiful Chalk fishes in the neighbouring quarries.
FOREIGN LOCALITIES.
⁂ Although the present work is expressly designed as a guide to the British collector, I am induced to subjoin a few foreign localities of Ichthyolites, that lie within the reach of the continental tourist. A detailed account of the most celebrated sites is given by M. Agassiz, Poiss. Foss.
Aix, in Provence. Tertiary. Some of the beds of gypseous marl contain numerous species in abundance.
Eisleben, Upper Saxony. Permian. Numerous Ichthyolites in dark shale.
Glaris, Switzerland. Cret. Immense numbers of fishes in dark schist. The specimens are often contorted, from the contraction of their bodies, during decomposition.
Maestricht (St. Peter’s Mountain). Upper Cret. Numerous teeth, vertebræ, &c. of fishes of the Cretaceous epoch. See Wond. p. 309. Mansfeld, in Thuringia. Permian. Fishes in copper-slate, in great numbers; many extremely beautiful.
Monte Bolca, or Vestena Nova. Tert. The richest mine of Ichthyolites in the world. A catalogue of the numerous genera and species found in this celebrated locality, is given in Poiss. Foss. tom. iv. pp. 33-52.[573] See Wond. p. 265.
Mount Lebanon, Asia. Tert. Numerous Ichthyolites, in great perfection.
Œningen. Tert. fresh-water. Many kinds of fishes of the same genera as those which inhabit the great European lakes; as the Perch, Salmon, Eel, Pike, Carp, &c. A list of these Ichthyolites will be found in Poiss. Foss. tom. ii. part ii. p. 78. See Wond. p. 263.
Saarbrück, in Lorraine. Carb. Amblypterus, and other Carboniferous fishes.
Seefeld, in the Tyrol; on the principal road from Insbruck to Munich. Lias. Abundance of fish in bituminous slate.
Stabia, Italy, at Torre d’Orlando, near Castellamare. Oolite. Beautiful fishes in fissile limestone.
Solenhofen. Oolite. Numerous Ichthyolites; many in great perfection. See Wond. p. 513.
[573] It is necessary to caution the collector against the frauds practised by the quarry-men, and dealers in fossils, at this and other celebrated foreign localities. Specimens, apparently perfect, are ingeniously constructed from the fragments of various examples. The head of one fish, the body of another, decorated with the fins of a third, and perhaps the tail of a fourth, of different species, or even genera, are dove-tailed together, coloured, and varnished, so as to deceive the common observer, and, occasionally, even the experienced collector. Sponging the specimens with cold water will often detect the imposition; for the colour if artificial will be removed, or rendered paler, while the same process will heighten the natural tints. At Pappenheim, Solenhofen, and other places, where fossil crustaceans, as Shrimps, Prawns, &c. are found in such perfection, the imprints of good specimens are often coloured, and offered for sale; a wet sponge will speedily detect the imposture.Return to passage
Chapter XVI
FOSSIL REPTILES; COMPRISING THE ENALIOSAURIANS AND CROCODILES.
"Nous remontons done à un autre âge du monde; à cet âge où la terre n’étoit encore parcourue que par des reptiles a sang froid--où la mer abondoit en ammonites, en bélemnites, en térébratules, en encrinites, et où tous ces genres, aujourd’hui d’une rareté prodigieuse, faisoient le fond de sa population."--Cuvier, Oss. Foss. tom. v. p. 10.
We advance now to the investigation of the fossil remains of the more highly organized classes of the Vertebrata; the Fishes being the lowest in the scale amongst the beings characterised by an osseous skeleton, with a flexible spinal column, composed of articulated bones, and presenting, in the various classes, orders, genera, and species, numerous modifications of form and structure. The mineralized relics of the vertebrated animals consist, for the most part, of single and displaced bones, or groups of bones and teeth, and the durable portions of the dermal integuments; entire skeletons being of rare occurrence. A knowledge of anatomy and physiology, and access to anatomical and zoological libraries and collections, are therefore indispensable for the cultivation of this most attractive department of Palæontology. Fortunately for the English student, this branch of the science, which a few years since was but little cultivated in this country,[574] has been greatly advanced, by the liberal support afforded by the British Association of Science to Professor Owen, whose Reports on the British Fossil Reptiles and Mammalia, published in the Transactions of the Association,[575] should be referred to for more precise and detailed information than can be given in these unpretending volumes. Our remarks will be limited to a general notice of the fossil remains of Reptiles, Birds, and Mammals; with descriptions of such characteristic examples, as will serve to illustrate the nature of the specimens that may probably come under the notice of the collector; or which, from their peculiar characters, are objects worthy his special attention.
[574] See Petrif. p. 226, note.
[575] Report of the Brit. Assoc. 1839 and 1841; see also Trans. Geol. Soc. 2d ser. vol. v. p. 515 (1838).
The Age of Reptiles.[576]--The announcement by the illustrious founder of Palæontology, in the quotation prefixed to this chapter, that there was a period when the lakes, rivers, and seas of our planet were peopled by reptiles, and when cold-blooded oviparous quadrupeds, of appalling magnitude, were the principal inhabitants of the dry lands, was a proposition so novel and startling, as to require the prestige of the name of Cuvier to obtain for it any degree of credence, even with those who were prepared to admit that a universal deluge could not account for the physical changes, which the crust of the earth had evidently undergone. Subsequent observations and discoveries have, however, fully confirmed the truth of this induction, and the "Age of Reptiles" is no longer considered fabulous.
[576] "The Age of Reptiles" was the title given by the author to a popular summary of the evidence bearing on this question: it was published in the Edinburgh Philosophical Journal, 1831. This name is now generally employed to designate the geological epochs characterised by the predominance of oviparous quadrupeds; namely, from the Permian to the Chalk, inclusive.
In some of the ancient fossiliferous deposits,[577] indications of the existence of Reptiles are visible, in the indelible markings left by their footsteps on the muddy banks of rivers, and on the wet sands of the sea-shores, now in the state of layers of marl and sandstone. Here and there in the Devonian,[578] Carboniferous, and New Red formations, teeth and bones are found, presenting unequivocal proofs of the presence of extinct forms of cold-blooded oviparous quadrupeds. As we ascend in the secondary formations, we are suddenly surrounded by innumerable marine and terrestrial reptiles, belonging to species and genera, and even orders, of which no living representatives are known. Throughout the Liassic, Oolitic, Wealden, and Cretaceous epochs, the class of Reptiles was at its fullest development. In the Tertiary periods which succeeded, the Reptiles approach the recent types, and their relics are found intermingled with the bones of mammiferous quadrupeds; thus indicating the commencement of the present condition and relations of the animal kingdom. Referring the reader to Bd. p. 165, and Wond. pp. 409-444 and 567-588, for a more comprehensive view of this subject, we advance to the examination of some of the fossil genera and species; and we propose, in the first place, to explain a few essential characters of form and structure observable in those durable parts of the skeletons which are most frequently met with in a fossil state; namely, the teeth, jaws, vertebræ, &c., and the osseous appendages of the dermal system.
[577] Devonian rooks of Elgin, North Britain; and the Lower Carboniferous of Pottsville, Pennsylvania.
[578] The most ancient Reptile hitherto discovered is the Telerpeton Elginense, from the Old Red of Scotland, which will be described in the sequel.
The animals comprehended in the Class of Reptilia constitute, according to Prof. Owen’s arrangement, eight principal groups, or Orders, as follow:--
The Batrachia; Frogs: the body naked, } with only rudimentary ribs; and with } two or four feet. Most of these reptiles } breathe by branchiæ or gills in their } young state, and by lungs in the adult } (as for example the Frog); in some (the } perenni-branchiata), the branchiæ are } persistent through life. } Having a tripartite } heart (i. e. with The Ophidia; Serpents: the body } two auricles and destitute of feet. } one ventricle), and } simple transverse The Lacertia or Sauria; Lizards: the } processes to the body supported by four or two feet, and } cervical and anterior covered with scales. } dorsal vertebræ. } The Chelonia; Tortoises: the body } supported by four feet or paddles, and } enveloped in two osseous bucklers, } composed of the expanded bones of the } sternum and thorax. }
The Enaliosauria; Sea-saurians } (extinct): body furnished with four } paddles, and destitute of scaly covering. } } The Pterosauria; Wing-saurians } (extinct): body supported on four } Having a quadripartite feet, the outer finger of each fore-foot } heart (i. e. with two greatly lengthened, and forming a support } auricles and two for the wing. } ventricles),and double } transverse processes to The Crocodilia; Crocodiles: body } the cervical and anterior supported on four partially webbed feet, } dorsal vertebræ. and encased with an armour of bony plates } or scutes. } } The Deinosauria; Great-saurians } (extinct): body supported on four feet. }
Teeth of Reptiles.--The teeth of the animals of this class exhibit considerable diversity of form, but the characteristic type is that of a conical, pointed tooth, with a simple root or fang; for, in no reptile does the base of the tooth terminate in more than one fang, and this is never branched. "Any fossil, therefore, which exhibits a tooth implanted by two fangs in a double socket, must be mammiferous, since the socketed teeth of reptiles have but a single fang; and the only fishes’ teeth which approach such a tooth in form, are those of a bifurcate base, belonging to certain sharks." (Owen.)
These dental organs are only fitted for seizing and retaining the prey or food; for no living reptiles have the power of performing mastication. In the Crocodiles the tooth has a cylindrical shank, with a conical, longitudinally striated, enamelled crown, having a ridge on each side (Pl. VI. fig. 5). In the Labyrinthodon (a fossil reptile), the cone is more curved and pointed (Pl. VI. fig. 3); in the Hylæosaurus, the shank is cylindrical, and the crown expanded and lanceolate, with blunt margins (Pl. VI. fig. 6); in the Megalosaurus, the tooth is laterally compressed, trenchant, and slightly inclined backwards like a sabre, with serrated edges (Pl. VI. fig. 7); in the Iguanodon, the shank is cylindrical, and the crown of a prismatic form, greatly expanded, with broad denticulated edges, and longitudinal ridges on one side (Pl. VI. fig. 4, and Ligns. 221, 223). In the Serpents, the teeth are very long and pointed; in the Crocodiles and Lizards, may be seen every modification of the conical form, down to a mere hemispherical tubercle or plate. In the fossil Dicynodon, to be hereafter described, the dental system consists of but two tusks or canine teeth, like those of the Walrus, implanted in the upper jaw. The Turtles and some fossil Lacertians are edentulous, i. e. destitute of teeth; their dental organs consisting of the horny trenchant sheaths with which the jaws are covered.
The teeth are very numerous in reptiles; the individuals of some species have more than two hundred. In some genera, they are implanted on the jaws alone; in many, they occupy the palatine, vomerine, and other bones composing the vault of the mouth, as in certain fishes. The teeth are generally anchylosed to the bone; but in some genera they are implanted in distinct sockets, as in the Crocodile and Plesiosaurus; in others, as in the Ichthyosaurus, they are arranged in a deep furrow, and retained only by the integuments; in some, they are supported upon an elevated osseous base. In the Labyrinthodonts, and in the greater part of the Serpent tribes, the tooth is implanted by the base in a shallow socket, with which it is confluent.
In most of the Lacertians, or true Lizards, the attachment of the teeth presents a peculiar modification, of which the lower jaw of the Iguana, Lign. 205, p. 649, affords a good illustration. The teeth are not placed in sockets, but are attached by the shank to an alveolar plate, or parapet, that extends along the margin of the jaw, as shown in figs. 1 and 3; the crowns of the teeth project above this plate, as seen in figs. 2 and 4. From the anchylosis of the teeth to the side of the jaw, the Lizards possessing this dental structure are termed Pleurodonts.[579]
[579] The Pleurodonts are those lizards in which the teeth are anchylosed to the side of the dentary bone; Acrodonts, those with the teeth fixed to the upper margin or ridge of the jaw-bone; Thecodonts, those having the teeth implanted, either loosely, or anchylosed to the walls of their sockets. Lacertians are also said to be Pleodont (having solid teeth), or Cœlodont (hollow-toothed).
In reptiles, we have, therefore, five essential modifications in the attachment of the teeth; namely, in distinct sockets; in a continuous groove or furrow; attached laterally by the shank to an alveolar parapet; anchylosed by the base to a shallow socket; and attached to an osseous support, without sockets or an alveolar plate.[580]
[580] See the beautiful exemplification of this subject, and the comparison between the transitory stages of the human teeth in their progress of development, discovered by Mr. Goodsir, with the permanency of these conditions in reptiles. Odontography, p. 182.
Fig. 1.--The right branch of the lower jaw of an Iguana; viewed on its inner aspect. Nat. size. a. Dentary bone. b. Opercular bone. c. Complementary or coronoid bone. d. Surangular bone. e. Angular bone. f. Articular bone. 2.--The external aspect of the same. 3.--Inner aspect of three teeth (magnified) attached to the alveolar parapet; with the germ of a successional tooth at the base of the middle tooth; and the sockets of other germs at the bases of the outer two fully formed teeth. 4.--External view of the crowns of three teeth; slightly magnified.
The intimate structure of the teeth consists of a simple pulp-cavity, surrounded by dentine, which is permeated by extremely minute calcigerous tubes, radiating at right angles to the periphery, or external surface of the tooth. One essential modification of this structure consists in the intermingling of cylindrical processes of the pulp-cavity, in the form of medullary or vascular canals, with the finer tubular structure; as in the tooth of the Iguanodon, Pl. VI. figs. 4^b and 4^c. But another modification is that to which allusion was made when describing the teeth of the Lepidosteus (see p. 616); in this mode, the dentine preserves its normal character, but the external cement and surface of the tooth are deeply inflected in longitudinal folds around the entire circumference; and this structure is accompanied with corresponding extensions of the pulp-cavity and dentine into the interspaces of these inflected and converging folds.[581] This organization is shown, in its simplest form, in the transverse section of the base of a tooth of the Ichthyosaurus, Pl. VI. fig. 9; and attains its most complicated condition in that of the Labyrinthodon, Pl. VI. figs. 3^a, 3^b, 3^c.
[581] There is a marked difference between the internal structure of the teeth of true Saurians and of Sauroid Fishes. In the former, as well as in the Enaliosauria, the dentine consists of tubes radiating from a slender central pulp-cavity to the periphery of the tooth, without any intermixture of vascular canals. In the sauroid fish (Dendrodus) the central pulp-cavity is produced into numerous irregular canals, from which vascular sinuses radiate to the periphery, sending off branches generally at right angles throughout their entire course; thus, there is an extensive distribution of the vascular system through the body of the tooth, which does not exist in any saurian reptile; the nearest analogy is in the labyrinthine teeth of the gigantic fossil batrachians. (Owen: Odontography; and Art. Teeth, Cyclop. Anat.)
With regard to the mode of development of the teeth, we must briefly state, that the germ of the new tooth is always produced at the side of the base of the old one; that in its progress of growth it presses against the tooth it is destined to supplant, occasions the progressive absorption of the fang, and ultimately displaces its predecessor; in some instances, by splitting the crown of the tooth; in others, by casting it off, according to the oblique or direct position the new tooth attains in its progress, in relation to its predecessor. Thus, in the teeth of the Crocodile, the new tooth is generally found immediately under the conical apex of the crown, and beneath the former a second successional tooth appears, like a series of thimbles of various sizes placed one upon another; for in reptiles the production of new teeth is unlimited. But in the Pleurodont lizards, the new tooth makes its way obliquely, and the crown is often shed entire. Lign. 205, fig. 3, exemplifies the situation of the successional teeth in the Iguana.
Lower Jaw of Reptiles.--It is well known that the lower jaw in mammiferous animals is composed of a single bone on each side; and that in many genera these pieces become united in front, and form but one bone in the adult state. But in reptiles, the lower jaw consists of six distinct bones on each side, as in Lign. 205; and these undergo various modifications of form and arrangement in the different genera. These bones are distinguished by names which have reference to their office and situation, and are as follow:--Lign. 205, a, the dentary bone, supporting the teeth; b, the splenial or opercular; c, the coronoid or complementary; d, the sur-angular; e, the angular; f, the articular, which forms the upper portion of the jaw, and includes the condyle. The form and disposition of these bones in the Iguana, and other true lizards, are shown in Lign. 205; but they differ materially in the Crocodile, Ichthyosaurus, and other genera. We must restrict our comments to this short notice, which, however, will suffice to enable the collector who discovers a fragment of a lower jaw, with any traces of the structure above described, to determine that it is reptilian; and if any portion of the dentary bone remains, indications may be obtained of the family, and perhaps genus, to which it belonged.[582]
[582] To obtain a correct knowledge of the osteological structure of fossil Reptiles, the student should consult Baron Cuvier’s Ossemens Fossiles, tom. v. To the English reader, the translated abridgement of Cuvier’s "Fossil Remains of the Animal Kingdom," by E. Pidgeon, 1 vol. 8vo. with plates, 1830, will be found a very instructive volume. See also Penny Cyclopædia, Art. Saurians.
Vertebræ of Reptiles.--The bones of the vertebral column of this class of animals present such numerous and important modifications in the different orders and families, that reference to the works already cited must be made for satisfactory information on this topic. From the great number of vertebræ in many reptiles, amounting in the individuals of some species to nearly two hundred, these bones are the most abundant fossil relics of these animals to be found in our collections. The vertebræ are commonly detached, and deprived of their processes; the solid centrum, or body, alone remaining in most examples (as in Lign. 206, fig. 8). Connected series, more or less complete, are occasionally discovered; and the entire column, in connexion with other parts of the skeleton, is preserved in many specimens in the British and other museums.[583] Although, for the reasons previously stated, minute osteological details cannot be attempted in this work, some acquaintance with the elementary characters of the bones composing the spinal column, and of the nomenclature employed to distinguish them, is necessary to guide the student, and even the amateur collector, in their researches. I have, therefore, selected a few specimens from Tilgate Forest in illustration of the elements of Saurian vertebræ, and of the terms by which the different processes are distinguished; the general reader will thus be enabled to comprehend the descriptions of these structures in other works on Palæontology.
[583] See Petrifactions, pp. 136-352, and pp. 362-387.
The bones composing the spine, are not only designed to form a flexible column of support to the trunk, but also to afford protection to the grand nervous chords constituting the spinal marrow, and which extend from the brain to the tail, and give off numerous lateral branches in their course, conferring sensation and motive power to every part of the frame. To effect this purpose, there is attached to the upper or dorsal part of each vertebra a bony ring, called the neural-arch, which is composed of two processes (Lign. 206, b.), arising from each side of the body or centrum (Lign. 206, a.), and which unite above into a solid piece, termed the spinal process, ox neural spine (Lign. 206, d.). On each side of the annular paid there is a process, called the transverse (Lign. 206, e, e.), for the attachment of muscles; and in the middle and the posterior dorsal regions of some reptiles, as, for example, in the existing Crocodiles, these processes articulate with the ribs. The vertebræ of the tail have, in addition to the above, an inferior spinous process, termed the chevron-bone (Lign. 206, fig. 2, and fig. 3, f.), which gives support to the inferior layers of the caudal muscles; and, bifurcating at its attachment to the body of the vertebra, leaves a channel for the passage of the large blood-vessels, by which the circulation of the tail is effected.
In the generality of living reptiles (as, for example, in the Crocodile) the bodies of the vertebræ are concave in front, and convex behind; the bones of the spine being united by ball-and-socket joints; but, in most fossil reptiles, both faces are either flat, or more or less concave. In mammalian quadrupeds, the annular part is anchylosed to the vertebral centre; but in reptiles, it is united by suture, although, in old subjects, the connecting line is often obliterated. By reference to Lign. 206, and its description, the form, arrangement, and connexion of the different vertebral elements, in certain fossil reptiles, may be easily understood. The bones in the vertebral column of the same animal are considerably modified in the several regions of the neck (cervical vertebræ), back (dorsal and lumbar), and tail (caudal). The cervical are generally of the most complicated structure; and the caudal, the most simple.
From this exposition, the reader will perceive that every vertebra consists of the following essential parts: first, the body, or centrum; and secondly, the annular part, or neural arch, so named, because it protects the nervous chord; while a caudal vertebra has, in addition, the chevron-bone, called also the hæmal arch, from its affording a passage to the large blood-vessels. The bodies of the vertebræ are in general solid, and consist of the ordinary osseous structure; but in certain fossil vertebræ the centre of the bone is filled with calcareous spar, indicating an irregular medullary cavity, as in the caudal vertebræ of the Ox.[584]
[584] See Petrifactions, p. 166, note.
The Sacrum, which may be termed the key-stone of the pelvic arch, is formed in existing reptiles by the union of two vertebræ; but in the Iguanodon and the Hylæosaurus the sacrum is composed of six anchylosed vertebræ; in the Megalosaurus probably of but five.[585]
[585] Report, Brit. Assoc. 1841, p. 105, and p. 130.
From the sides of the two anchylosed vertebræ which form the sacrum, strong, short, rib-like processes are given off in those Saurians which occasionally walk on dry land, and these constitute a firm support to the hinder extremities.
In the Crocodiles, the four or five vertebræ preceding the sacrum have no ribs attached to them, and are termed lumbar; in the Lizards, there are but two lumbar vertebræ. A peculiar modification exists in the first caudal vertebra of the adult Gavial and Crocodile; the centrum is convex both in front and behind, as was first demonstrated by me in 1836. See Wond. p. 419, and Petrif. p. 167. The last of the anchylosed vertebræ forming the sacrum is concave posteriorly; hence the necessity of an anterior ball in the first joint of the tail. (See Lign. 217, p. 676, illustrative of Crocodilus Hastingsiæ.) The last cervical vertebra in the Turtles and Tortoises has a similar construction. This mechanism confers freedom of motion without risk of dislocation.
Ribs.--The Ribs, which are regarded as appendages to the vertebræ, (homologues of the pleurapophyses,) are generally slender and round in the Lizards, and articulate with the spinal column by a single head, supported on a short convex process or tubercle. In Crocodiles only of all existing Reptiles, but in several extinct genera, the proximal end of the rib forms a double articulation, by a distinct head and a tubercle, with the vertebræ in the cervical and anterior dorsal region of the spinal column; in the posterior dorsal region the ribs are attached to the elongated transverse processes of the vertebræ.
As this double articulation of the ribs is invariably associated in existing reptiles with a heart having double ventricles, while the lacertian single-headed ribs are in like manner connected with a heart having but one ventricle, the student will perceive the important physiological inferences that spring from the discovery of a mere fragment of a rib, when interpreted by the profound anatomist.[586] In some fossil reptiles the ribs are flat and very broad; as, for example, in the Hylæosaurus.[587]
[586] See Brit. Assoc. Report, 1841, and Memoirs, Palæont. Soc.
[587] In Crocodiles the abdominal region is strengthened by slender ribs (hæmapophyses, Prof. Owen), that are affixed to a ligamentous extension of the cartilaginous sternum, analogous to the linea alba in man; and the Hylæosaurus appears to have possessed a similar development of the costal elements, for I observed many fragments of long, slender, sub-cylindrical rib-like bones whilst chiselling off the stone from this species, and portions of similar bones occur in the stone around the spinal column from Bolney. In the Maidstone Iguanodon there are likewise some long slender bones of this character, which I think must be prolongations of the ordinary dorsal ribs.
Extremities.--The locomotive extremities are variously constructed, according to the adaptation of the animals to a terrestrial, fluviatile, or marine existence. The bones of the limbs in the extinct colossal terrestrial species much resemble those of our large pachydermata, the Rhinoceros and Hippopotamus. The cylindrical bones of the extremities in the Crocodilians, and other recent reptiles, are solid. i. e. have no cavity filled with marrow; such also is the case in the fossil Enaliosaurians; but the thigh-bones and leg-bones of the Iguanodon, and of other extinct land saurians, h ave a large medullary canal. Our limits will not admit of further osteological details; and we are compelled to omit the description of the bones composing the thoracic and pelvic arches.
Dermal Bones. Ligns. 207 and 208. In many of the reptile tribes, particularly of the Crocodilian or loricated (mailed) group, there are immediately under the external integument or skin a series of osseous scutes, or scutcheons, variously arranged, which serve as supports to the integumental scales and spines. In the gigantic Gavial, that inhabits the Ganges and other rivers of India, and which is remarkably distinguished from the common Crocodile and Alligator by an extremely elongated slender muzzle, the nape of the neck is protected by sixteen or eighteen transverse rows of dermal scutes; and there are likewise six rows which extend down the back. These bones are deeply corrugated or sculptured on their upper surface; a structure adapted for the firm adhesion of the horny integument.
Detached bones of this character occur in the Purbeck strata; and the first fragments I collected were supposed by me to belong to the soft-skinned turtles (Trionyces); but the subsequent discovery of perfect scutes demonstrated their analogy to the dermal bones of the Gavial, and enabled me to determine their true character.
In the splendid specimen of the fossil remains of a Crocodilian reptile (Goniopholis), found at Swanage (Wond. pp. 415; and Petrif. p. 170), there are numerous dermal examples dispersed among the bones, as shown in Petrif. Lign.
One of these is figured Lign. 207; fig. 1 represents the external surface, which is deeply sculptured by irregular roundish pits or excavations; the under or inner surface, fig. 2, is smooth, but marked with very fine striæ, decussating each other at right angles, as in the dermal bones of the Hylæosaurus (Lign. 208, fig. 1a.). These scutes differ from those of other recent and fossil Crocodilians, in a lateral conical projection, marked a, _figs. 1, 2, Lign. 207, which fits into a depression on the under surface of the opposite angle of the adjoining plate; resembling, in this respect, the scales of the Lepidotus (see Lign. 196, p. 605). Numerous hexagonal and pentagonal scutes, articulated together by marginal sutures, also entered into the composition of the osseous dermal cuirass of this reptile, which must, therefore, have possessed a flexible, yet impenetrable, coat of armour, capable of affording protection against the attack of any assailant.
In the Oolite, the dermal bones of other slender-nosed Crocodilians (Teleosaurus) are occasionally met with; the outer surfaces of which are marked with small circular distinct pits; these scutes are thicker and more rectangular than those above described, and slightly overlapped each other laterally; they have no connecting process. In another species one half of the outer surface is smooth, proving that it was covered to that extent by the adjoining scute.[588]
[588] A description of the dermal bones of British fossil reptiles is given in Brit. Assoc. Report for 1841, pp. 70, 79, &c.
Dermal Bones of the Hylæosaurus.--Elliptical and circular dermal scutes, having the under surface flat and the upper convex with a conical tubercle, were first noticed in the specimen of the Hylæosaurus, figured Wond. pl. iv.; and I have since discovered similar bones associated with other remains of that extraordinary reptile; reduced figures of two specimens are represented in Lign. 208, figs. 1, 3.
The structure of these bodies is very remarkable; upon closely inspecting the under side, and the surface exposed by a transverse fracture, very minute osseous spicula, decussating each other at right angles, are distinctly seen; as shown in Lign. 208, fig. 1a. In fig. 1b, a thin slice of the same, highly magnified, and viewed by transmitted light, displays medullary canals, with very fine lines radiating from them. The peculiar character of this organization consists in the disposition of the straight bony spicula; an appearance which first attracted my attention when developing the original specimen of the Hylæosaurus (see Geol. S. E. p. 327), and led to the discovery of some perfect examples, which otherwise would have been destroyed. This structure closely resembles that presented by the ligamentous fibres of the corium, or skin, and seems to have resulted from an ossified condition of the dermal integument. These bones vary from half an inch to three or four inches in diameter, and were disposed in one or more longitudinal series on each side the spine, diminishing in size as they approach the end of the tail.
Dermal Spines of Hylæosaurus. Lign. 208, fig. 4.--With the dermal bones above described there are associated in the first discovered specimen of the Hylæosaurus, flat, thin, angular, osseous plates, from three to seventeen inches in length; one of which is figured Lign 208, fig. 4_. The manner in which they are imbedded in the rocks, in connexion with other parts of the skeleton, is shown Wond. pl. iv. and Geol. S. E. pl. v. These very remarkable processes appeared to me to have formed part of a serrated fringe, which extended along the back of the reptile, analogous to that observable in certain living lizards (Wond. p. 436, Lign. 108); and were provisionally described as such in my first memoir on the Hylæosaurus. This conjecture has been substantiated by subsequent discoveries, and the true nature of the large, flat, angular spines, and the conical bones resembling the horn-cones of ruminants, which occur in the Wealden, is now established.[589]
[589] See Fossils, Brit. Mus. pp. 298, 320.
Horn of Iguanodon.--In this category may be placed the nasal tubercle or horn of a saurian, like that of the Iguana (Lign. 208, fig. 2; Geol. S. E. pl. iii.), found with the remains of the Iguanodon, and probably belonging to that colossal reptile (Wond. p. 431; and Petrif. p. 298). It is four inches in length and 3.2 inch by 2.1 inch in diameter at the base, which is of an irregular elliptical form. Several smaller specimens have recently been discovered.
Examples of dermal scutes and spines, presenting modifications of form and structure distinct from those above described, have been brought to me from various localities of the Wealden; but, as in no instance a connexion with other parts of the skeleton could be traced, the particular reptiles to which they belonged cannot be ascertained.
We proceed to notice some of the principal genera of Fossil Reptiles, especially of those whose remains occur in the British strata; the arrangement of Professor Owen is adopted for the convenience of reference to the Brit. Assoc. Reports, 1839 and 1841, which should be consulted by the student who would acquire a knowledge of this department of Palæontology. The subject will be considered under the following heads; namely:--
I. Enaliosaurians, or Marine Reptiles; Ichthyosaurus and Plesiosaurus.
II. Crocodilians; Crocodile, Teleosaur, &c.
III. Deinosaurians; comprising the Iguanodon, Megalosaurus, Hylæosaurus, Pelorosaurus, &c.
IV. Lacertians; including the Mosasaurus, Rhyncosaurus, &c.
V. Pterosaurians; the Pterodactyles, or flying Reptiles.
VI. Chelonians; or Tortoises and Turtles.
VII. Ophidians; or Serpents.
VIII. Batrachians; or the Frog tribe; comprising the Labyrinthodonts.
I. Enaliosaurians.--The extinct marine reptiles comprised in this order constitute two genera, which are characterized by essential modifications of osteological structure; they are named Ichthyosaurus (fish-lizard), and Plesiosaurus (akin to a lizard). The general appearance of these beings is so well known, from the splendid collection of their fossil remains in the British Museum, and the numerous specimens in provincial and private collections, and by various works, both scientific and popular, in which their structure and physiological relations are fully elucidated, that they must be familiar to every reader.[590]
[590] Bd. vols. i. and ii. contain an admirable exposition of their habits and organization; and Brit. Assoc. Reports, 1839, 1841, elaborate osteological investigations of both genera. A folio volume on these extinct Reptiles, with splendid lithographs, by Thomas Hawkins, Esq., cannot fail to delight the reader by its graphic descriptions and beautiful illustrations. See also the masterly paper on the Ichthyosaur and the Plesiosaur, by the Rev. W. D. Conybeare, in the Geological Transactions, 1st series, vol. v. p. 559, et seq.
[Side note: ICHTHYOSAURUS. PLESIOSAURUS.]
The living Ichthyosaurus must have borne a resemblance to a Grampus or Porpoise, with four large flippers or paddles and a long tail, having a vertical caudal fin of moderate dimensions; the skin probably being naked and smooth, as in the Cetaceans. The Plesiosaurus presented a configuration still more extraordinary (Wond. p. 575). With a very small head, it possessed a neck of enormous length, a body of moderate size, with four paddles, resembling those of turtles, and a very short tail. They were both marine, air-breathing, cold-blooded, carnivorous, vertebrate animals; swarming in prodigious numbers during the secondary epochs, and particularly in the seas of the Liassic period (Ly. p. 277, figs. 310, 311). In both genera the construction of the skeleton presents many important variations from all known recent types; and should be carefully investigated by the student, who will find in the Reports of Professor Owen above referred to all the information that can be desired.[591]
[591] The Penny Cyclopædia, Art. Plesiosaurus, contains an able abstract of these Reports; and in the Fossils of the British Museum, the student will find a full account of the discovery of the Ichthyosaur and Plesiosaur, and of the deposits in which they are chiefly found, as well as detailed descriptions of the most characteristic structures of the different species.
It will suffice for our present purpose to point out a few important and obvious characters.
In the Ichthyosaurus, the nasal apertures or openings of the nostrils are not towards the snout, as in the Crocodile, but near the anterior angle of the orbit (see Lign. 209), approaching, in this respect, some of the recent lizards. The orbit is very large, and the sclerotic coat or capsule of the eye has in front an annular series of bony plates (Bd. pl. x. figs. 1, 3), which often occur in their natural position (Lign. 209). This structure is not possessed by fishes, but is analogous to that observable in the eyes of turtles, lizards, and many birds; as for example, in the owl and eagle: it confers on the eye additional power of adaptation and intensity of vision. The muzzle of the Ichthyosaurus is long and pointed; the lower jaw is formed of two branches, united anteriorly through nearly half their length; each branch is composed of six bones, as in the Crocodile and Lizards, but differently arranged than in those reptiles. The teeth are very numerous, amounting to nearly two hundred in some species, and are placed in a single row along the jaws, being implanted in a deep continuous groove (see Bd. pl. xi.). These teeth are of a pointed conical form, longitudinally striated, with an expanded base (Lign. 210). The new teeth are developed at the inner side of the base of the old, and grow up and displace them (see Lign. 210). The microscopical structure of the teeth of the Ichthyosaurus is beautifully illustrated by Professor Owen (Odontography, p. 275, pl. lxiv.). The tooth consists of a pulp-cavity, surrounded by a body of dentine, which is invested at the base by a thick layer of cement; and at the crown by a coat of enamel, also covered by a pellicle of cement 3 the pulp-cavity, in fully-formed teeth, is more or less occupied by coarse bone. The chief peculiarity of this structure consists in the inflection of the cement into vertical folds at the base of the tooth, by which the marginal portion of the basal dentine is divided into a corresponding number of processes; producing, in a transverse section, the appearance represented in Pl. VI. fig. 9. This organization, as we have previously remarked, is similar to that observable in the teeth of the Lepidosteus (see p. 616), and of the extinct reptile, called Labyrinthodon, hereafter to be noticed.
The vertebræ; (Bd. pl. xii.; and Lign. 211), of which there are upwards of one hundred and forty in the individuals of some species, are relatively very short in their antero-posterior diameter (i. e. from front to back); and deeply cupped on each articulating face, as in fishes. The annular part is not united to the body of the vertebra, as in mammals, nor connected by suture, as in Crocodiles, but terminates on each side in a compressed oval base, which fits into corresponding sockets placed on the boundary line of the spinal depression on the body (Lign. 210, 2a); thus completing the medullary canal (see Bd. pl. xii. fig. D, E.). Hence the collector may easily recognise the body of an Ichthyosaurian vertebra, by the pits or depressions on the sides of the spinal interspace. The first and second vertebra; are anchylosed together, and have additional subvertebral, wedge-shaped bones, which render this part of the column a fixed point of support.[592] (Bd. pl. xii. figs. 3, 6.) The form and arrangement of the bones that enter into the composition of the pectoral and pelvic arches, and of the paddles, are exemplified in Bd. pl. xii.; and full osteological details are given in Brit. Assoc. Rep. 1839, p. 104. The characters of the several bones composing the pectoral arch of the Ichthyosaur will be readily understood from the accompanying illustration. The structure of the pectoral arch of the Plesiosaur is also shown in an accompanying Lignograph, for the sake of comparison. The bones of a fore-paddle of an Ichthyosaurus are represented (Lign. 214, fig. 1). In some species each paddle consists of nearly one hundred bones. These locomotive extremities are very analogous in their osteological construction to those of the Cetaceans, but they are connected with the trunk by means of the glenoid socket formed by the scapula and coracoid, which are firmly united to the sternum; whereas in the Cetaceans the pectoral fin is only attached to a simple scapula, which is merely suspended in the muscles. This structure, together with the presence of a clavicle in the Ichthyosaurus (see Lign. 212), which is wanting in the Cetaceans,, indicates, in the opinion of Professor Owen, that this marine fish-lizard was capable of some degree of locomotion on the land; and that it might have resorted to the shore to deposit its eggs, or, like the Crocodile, to sleep. From the frequent occurrence of a dislocation or abrupt bend of the vertebral series of the tail, at about one-third of its length from the end, supposed to have been produced by the weight of a large fin, during the progress of decomposition, and from the terminal caudal vertebræ being laterally compressed, it is inferred that the Ichthyosaurus had a vertical fin at the extremity of the tail, which would thus be rendered a powerful instrument of progressive motion.[593] From the appearance of the Coprolites, which occur abundantly with the skeletons of these animals, it is obvious that the intestinal canal in the Ichthyosaurus was furnished with spiral valves, as in the Sharks; and the comminuted bones and scales in the coprolites prove that fishes constituted the principal food of these marine reptiles.
[592] This structure was first demonstrated by Sir Philip Egerton. See Geol. Trans. 2d series, vol. v. p. 187, pl. xiv.
[593] Geol. Trans. 2d ser. vol. v. p. 511, pl. xlii.
The Ichthyosaurus has abdominal ribs (p. 656, note), as in the Crocodile, and it is therefore inferred that, if oviparous, it did not produce ova in such immense numbers as the Batrachians, &c. A specimen found by the late Mr. Channing Pearce renders it probable that the Ichthyosaurus may have been viviparous. A remarkably perfect adult Ichthyosaurus, examined by this gentleman, contained the bones of a fœtus (a few inches long) in the cavity of the pelvis. This specimen is in the collection of Mr. Pearce, at Bath. Remains or traces of the dermal integument have been discovered in some examples from the Lias of Barrow-on-Soar, Lyme Regis, Ilminster, and the neighbourhood of Tewkesbury.[594]
[594] See Mr. Coles’s interesting paper on the Skin of the Ichthyosaurus, in the Quart. Journ. Geol. Soc. vol. ix. p. 79.
Integuments of the Paddle.--The importance of carefully examining the surrounding stone before removing vegetable or animal remains from the matrix in which they are imbedded, and which has so often been insisted upon in the preceding pages, is strikingly exemplified in the highly interesting example of the hinder paddle of an Ichthyosaurus (I. communis) discovered by Sir Philip Egerton. Lign. 215 is reduced from the exquisite representation of the specimen accompanying the original memoir by Prof. Owen on this fossil, in the Geological Transactions. The specimen consists of the phalangeal bones of a posterior paddle, with the impression of the soft parts or integuments in their natural position; a, marks the termination or distal extremity of the fin, consisting entirely of the softer integuments; these gradually widen and expand to receive the terminal rows of the phalangeal ossicles or bones, marked b. The upper border of this integumentary part of the paddle (c) is formed by a smooth, well-defined line, apparently a mere duplicature of integument. But the lower margin (d) exhibits the impressions of a series of rays, by which the fold of integument was supported; these rays bifurcate as they approach the margin of the fin, and were probably either cartilaginous, or composed of an albuminous horny tissue, like the marginal rays in the fins of Sharks. Dr. Buckland detected remains of the dermal integument of an Ichthyosaurus in a specimen from the Lias at Barrow-on-Soar (Bd. ii. p. 22, pl. x.); and in a fine skeleton with the four paddles (now in the British Museum), which I obtained from that locality, there were decided traces of the carbonized integuments around each paddle, but which were, unfortunately, chiselled away, in developing the bones, before I was aware of their true nature.
In Mr. Coles’s paper, already referred to, the student has an instructive instance of the value of a careful examination of faint or obscure traces of organic matter accompanying these saurian remains, and how such an examination should be made. The Plate illustrative of the Memoir exhibits the minute, hooked, conical bodies, that form the dense felt-like mass which the black film, frequently accompanying these fossil bones, appears to consist of, when seen under the microscope. To what extent this substance entered into the constitution of the integuments, or of the exact relation of these "setiform scales" to the surface or the interior of the skin, our present knowledge does not enable us to judge.
[Sidenote: PLESIOSAURUS.]
Plesiosaurus. (Bd. pl. xvi.--xix.)--The animals of this genus present in their osteological structure a remarkable deviation from all known recent and fossil reptiles; uniting the characters of the head of a lizard, with the teeth of a crocodile, to a neck of inordinate length, with such modifications of the ribs, the pectoral and pelvic arches, and the paddles, as to justify the graphic simile of Professor Sedgwick, that the Plesiosaurus might be compared to a serpent threaded through the shell of a turtle.
The character which immediately strikes the observer, is the extraordinary length of the neck, and the relative smallness of the head. The neck, which in most animals is formed of but five vertebræ, and in the extremest recent example, the Swan, does not exceed twenty-four, is in the Plesiosaurus composed of from twenty to forty; and, in some species, is four times the length of the head, and equal to the entire length of the body and tail; while the length of the head (in P. dolichodeirus) is less than one-thirteenth of the entire skeleton. The skull resembles that of the crocodile in its general form, but is relatively smaller, and is more related to the lacertian type. The parietal bone is more triquetal than in the crocodiles; but the zygomatic bone is attached to its lower end. The breathing apertures are situated anterior to the orbits, on the highest part of the head. The lower jaw has the usual structure of the Saurians; but the dentary bone is greatly expanded anteriorly, and united in front (see Bd. pl. xix.). The teeth are implanted in separate sockets, as in the crocodile, and there are from thirty to forty on each side the jaws. They are conical, slender, long, pointed, slightly recurved, and longitudinally grooved from the base upwards; having a long round fang. The pulp-cavity is long and single, surrounded by a body of firm dentine, covered on the crown with a layer of enamel, and at the base with cement (Odont. pl. lxxiv.). The dentition in the Plesiosauri differs from that of the Crocodiles, in the successional teeth emerging through distinct apertures on the inner side of the sockets of their predecessors, and not through the pulp-cavity. The vertebræ are relatively longer than in the Ichthyosaurus, and their articular faces are either flat, or slightly excavated towards the periphery, with a gentle convexity in the centre (Foss. Til. For. pl. ix. fig. 4).[595]
[595] For details, see Brit. Assoc. Rep. 1839, p. 50.
The caudal vertebræ have two distinct hæmapophyses, not united into a chevron-bone.
The cervical ribs, or hatchet-bones, are attached by two articular facets to the bodies of the vertebræ, but with a very narrow space between; scarcely large enough even for the passage of the sympathetic nerve; and apparently not sufficient for the vertebral artery.
The pectoral arch is remarkable for the pair of elongated and broad coracoid bones (Bd. pl. xvii. and Lign. 213); indeed the coracoids attain their maximum of development in the Plesiosaurus. The ribs, which are very numerous, and extend throughout a great portion of the vertebral column, are connected, anteriorly, in the abdominal region, by several slender bones, called costal-arcs, consisting of six or seven pieces to each pair of ribs; the Ichthyosaurus has a similar structure, but the arcs are composed of but five pieces. As these connecting bones are so constructed as to admit of a certain degree of gliding motion upon each other, it is inferred that, by this mechanism, considerable expansion of the pulmonary cavities in these air-breathing marine reptiles was obtained (Bd. pl. xviii. fig. 3).
The paddles are composed of fewer and more slender bones than in the Ichthyosaurus, and must have been of a more elegant form, and possessed greater flexibility (Lign. 214, fig. 2). The wrist (carpus) consists of a double row of round ossicles, which are succeeded by five elongated metacarpal, and these by numerous, slender and slightly-curved phalangeal bones.
Sixteen species of Plesiosaurus and ten of Ichthyosaurus have been discovered in the British strata, and nearly forty are now known; their geological range is from the Lias to the Chalk, inclusive.[596] Their remains are found most abundantly in the Lias and Oolite. I have collected many Plesiosaurian vertebræ in the Wealden, and in the Green Sand of Farringdon. No traces of Ichthyosauri have been observed in the Wealden; but vertebræ, and jaws with teeth, occur in the lower Chalk and Galt of Kent and Cambridgeshire.[597] On the Continent the remains of Enaliosaurians have also been discovered in the same formations.
[596] See Petrifactions, for an account of the specimens in the British Museum.
[597] Brit. Assoc. Trans. 1845, Sect. p. 60. The Enaliosaurian bones and teeth found in the Cretaceous deposits of England have been fully described and illustrated by Prof. Owen, in Dixon’s Fossils of Sussex, &c., and in his Monograph on the Fossil Reptiles of the Cretaceous Formation, Palæontographical Society, 1851.
Pliosaurus.---This name designates a gigantic extinct reptile, of which the upper and lower jaws, with teeth, considerable portions of the vertebral column, and many bones of the extremities have been discovered in the Kimmeridge clay of Oxfordshire, and are preserved in Dr. Buckland’s museum. The teeth resemble those of the Plesiosaurus in their general aspect, being of a conical form, longitudinally grooved, and having a long fang; but they are readily distinguished by the subtrihedral form of the crown, produced by the smooth, flat, or slightly convex external surface; they approach in this respect the tooth of the Mosasaurus; from the latter, however, even fragments may be known by the presence of longitudinal ridges. The animal itself was an enormous marine reptile, allied to the Plesiosaurians, but more nearly related to the Crocodilians.[598]
[598] Brit. Assoc. Rep. 1841, p. 60. Odont. pl. lxviii.
II. Crocodilians.--The loricated, or mailed, Saurian reptiles, viz. the Alligators, Crocodiles, and Gavials, are well known as the largest living forms of cold-blooded oviparous quadrupeds.[599] No relics of any recent species have been observed in a fossil state; except that of the Gangetic Gavial, which has been found fossil in the Sub-Himalayas by Capt. Cantley and Dr. Falconer; but remains of Crocodilians of the existing generic type, having the spinal column composed of concavo-convex vertebra; (i. e. united to each other by a ball and socket-joint), the convexity being behind, or towards the tail, have been found in the London Clay at Hackney and the Isle of Sheppey, and in the eocene deposits on the coast of Western Sussex and Hants.[600] But the Crocodiles of the Wealden, Purbeck, Oolite, and Lias differ materially in their osteological characters from the recent species, particularly in the structure of the vertebral column; which in one genus is composed of concavo-convex vertebra; placed in a reversed position to those of the existing species, the ball or convexity being anterior, or directed forwards. In the other genera, both the articular faces of the vertebræ are either flat, or concave.[601] (Geol. S. E. p. 296.)
[599] A detailed and philosophical examination of the osteology of the recent Crocodilia has lately been given to the scientific world by Prof. Owen, in his Monograph on the Reptilia of the London Clay, published by the Palæontographical Society of London, 1850. A condensed notice, by Prof. Owen, of the dental apparatus of the Crocodilians, is to be found in the Cyclop. Anat. Art. Teeth.
[600] See Brit. Assoc. Rep. 1844, Sect. p. 50; and 1847, Sect, p. 65.
[601] Cuvier, Oss. Foss. tom. v. p. 153; on the fossil Crocodiles of Honfleur, which comprise both the types alluded to in the text.
Vertebræ of two species of Crocodilians or Alligators have been found in the cretaceous Green Sand of the United States: these are of the true procœlian[602] type, as in the existing species; but they present peculiar characters in the modification of the apophyses.[603]
[602] Procœlian, concave before: amphicœlian, concave at both ends platycœlian, flat in front and concave behind.
[603] See Quart. Geol. Journ. vol. v. p. 380, pl. x.
With this exception, the Crocodilians with broad muzzles, as the Cayman and Alligator, have no representatives below the Tertiary formations; the Crocodilia of the Secondary deposits being all referable to the division having elongated beaks, like the recent Gavials (Bd. p. 250). The fossil Crocodiles of the latter type are arranged in two genera; namely, Teleosaurus[604] (Lign. 216, fig. 3), in which the nasal apertures terminate in two orifices, (not blended into a single opening as in the recent species,) in front of the nose; and Steneosaurus (Lign. 216, fig. 2), in which the breathing canals end in two nearly semicircular vertical openings at the extremity of the muzzle. (See also Bd. pl. xxv.)
[604] The skeleton of a recent Gavial, and that of a Teleosaur, are represented on one plate, for comparison, in Prof. Owen’s Monograph, loc. cit.
The British fossil species, most nearly related to the recent, occur in the Lower and Middle Eocene of the South-East of England. Two fine specimens of the skull of Crocodilus toliapicus, Cuvier and Owen (C. Spenceri of Dr. Buckland), have been found at Sheppey, as well as a skull of C. champsoïdes (Owen), and numerous vertebræ referable to each species. The eocene deposits of Hordwell Cliff have yielded the Crocodilus Hastingsiæ,[605] and the Alligator Hantoniensis (Petrif. p. 467; and Charlesworth's Geol. Journ. pl. i.); and the remains of a Gavial (Gavialis Dixoni, Owen, in Dixon’s Foss. Suss.) have been found at Bracklesham.[606]
[605] This fossil Crocodile supplies a good illustration of the biconvex body of the first caudal vertebra already described as peculiar to these loricated reptiles, see page 656; and I have subjoined Lign. 217 in illustration of this structure.
[606] Most of these valuable fossils are in the British Museum. They are all described in detail and most elaborately illustrated in Prof. Owen’s Monograph, already referred to.
In the strata of Tilgate Forest, associated with innumerable remains of reptiles of various kinds, teeth of the Crocodilian type, belonging to two genera, are not uncommon.[607] The first kind (Suchosaurus[608] cultridens of Prof. Owen) is a tooth about an inch in length, of a slender acuminated form, compressed laterally, and gently recurved, with a sharp edge in front and behind; resembling, in its general figure, the tooth of a Megalosaurus, with the serrations on the edges worn off (Pl. VI. fig. 7). The sides of the crown are marked with a few longitudinal grooves. Some biconcave vertebræ found in the same quarries, and characterized by the compressed wedge-shaped form of the centre (Foss. Til. For. pl. ix. fig. 11), are supposed by Professor Owen to belong to the same reptile as the teeth above described; but it is hazardous to pronounce on the identity of these detached teeth and bones, without more corroborative proof than has hitherto been obtained.
[607] Foss. South Downs, p. 50. Foss. Tilg. For. p. 64; pl. v. figs. 1-3, 7. Cuv. Oss. Foss. tom. v. p. 161, pl. x. See also Owen's later examination of these remains, Report Brit. Assoc. 1811, p. 67; and Cyclop. Anat. Art. Teeth.
[608] Brit. Assoc. Rep. 1841, p. 68. Sucho-saurus is derived from Suchus, or Suchis, the name given by Strabo to the sacred crocodile of the Egyptians.
Swanage Crocodile. (Goniopholis crassidens.) Petrif. p. 170, Lign. 38.--Under this name, the second species of Crocodilian teeth will be considered; the discovery of a considerable portion of a skeleton of a reptile with teeth of this form (Wond. p. 416), in a quarry near Swanage, having disclosed some of the most important osteological characters of the original. These teeth are distinguished from the former by their cylindrical base, and rounded, obtuse, conical crowns (Petrif. p. 171): they somewhat resemble in form those of the Crocodile, but the crown is strongly marked with numerous, well-defined, longitudinal grooves and ridges; and there is a sharp ridge on the middle of each side. A small specimen, broken off at the base, is represented Pl. VI. fig. 5; it shows the smooth cylindrical base of the tooth, which is covered with cement, and the finely striated enamelled crown; some of the teeth are more than two inches in length, and one inch in diameter at the base (Foss. Til For. pl. v. figs. 1, 2). I have found these teeth in numerous localities; they are always well preserved, with the ridges sharp, and have a high polish (Geol. I. Wight, p. 357, Lign. 30); a series of successional teeth may often be detected in the pulp-cavity (see Wond. p. 414). The detached teeth, and fragments of dermal bones (Lign. 207), which, from their constant occurrence with this species, I had been led to consider as belonging to the same reptile, were the only relics that had come under my observation, until the discovery of the Swanage specimen above mentioned.[609] On the two corresponding slabs containing this fine fossil are imbedded many detached teeth; a portion of the left side of the lower jaw, with two teeth in place; ribs and numerous vertebræ, which are biconcave, and have an irregular medullary cavity in the centre of the body; chevron bones resembling those of the Crocodile; the bones of the pelvic arch, and some of those of the extremities. With these are the remains of the osseous dermal cuirass, consisting of numerous scutes (figured and described p. 657, Lign. 207), scattered at random among the other relics of the skeleton; some having the inner, and others the external surface exposed; several of these bones are perfect, and exceed six inches in length, and two and a half in breadth. Numerous scales of a small Ganoid fish (Lepidotus minor), common in the Purbeck strata, are also intermingled with these remains. This reptile is named Goniopholis crassidens, by Professor Owen.[610]
[609] Now in the British Museum: see Petrif. p. 170. A lithograph of one of the slabs was given in the third edition of the Wonders of Geology, 1839.
[610] Gonio-pholis:--angle-scute. Brit. Assoc. Rep. 1841, p. 72. Both the slabs of the Swanage specimen are admirably arranged in the same case in the British Museum, with bones and scutes of this reptile from Tilgate Forest; there can be no doubt that the entire lower jaw of the Swanage reptile might have been obtained if the quarry-men had taken the precaution of examining the adjoining block of stone.
Pœcilopleuron.--The remains of an allied genus of Crocodile, the Pœcilopleuron Bucklandi (of Deslongchamps), occur in the Oolite, near Caen, Normandy. This reptile, like the Goniopholis, had biconcave vertebræ, with a large medullary cavity in the middle of the centrum. The body of the vertebræ is contracted in the middle, the neural arch anchylosed, with no trace of suture, and with a thin spinous process, which is remarkable for its backward inclination. Vertebræ of this character also occur in the Wealden strata of Tilgate Forest and the Isle of Wight.[611]
[611] Brit. Assoc. Rep. 1841, p. 84. Foss. Til. For. pl. ix. fig. 8, represents a caudal vertebra.
Teleosaurus. (Bd. pl. xxv.)--In the Oolite of England and the Continent, the remains of a genus of extinct reptiles, having, like the recent Gavial, long slender muzzles, have been discovered in several localities. These fossils consist of the osseous scutes of an imbricated dermal cuirass; of the cranium and jaws with teeth; of the vertebral column; and many other bones. The characters of the dermal scutes, and of the muzzle with its terminal nasal apertures, have already been described (p. 659, and p. 676). There are several species of Teleosaurus; a splendid specimen of T. Chapmanni, fifteen feet long, from the Lias-shale on the Yorkshire coast, is preserved in the Whitby Museum; and there are interesting examples in the British Museum.[612] Teleosaurian remains have been found in the Oolite at Stonesfield, and at Deddington, Oxfordshire; and Mr. C. Moore, of Ilminster, whose museum is rich with perfect Ichthyosaurs and Fishes from the Upper Lias of the neighbourhood, has been highly successful in developing some charming specimens of small Teleosaurs from out of the same deposit. In the Oolite of Caen, in Normandy, very fine specimens of T. Cadomensis have been discovered; and from these the illustrious Cuvier first determined the character and affinities of the original.[613] The British Oolite contains also the relics of a reptile with biconcave vertebræ, belonging to the genus Steneosaurus: the cranium with the jaws and teeth have been found in Kimmeridge clay, at Shotover (Bd. pl. xxv.).[614]
[612] Petrifactions, p. 178.
[613] Oss. Foss. tom. v. p. 127.
[614] Report Brit. Assoc. 1841, p. 82.
From the Jura limestone at Monheim, in Franconia, the remains of a small Crocodilian reptile (Teleosaurus priscus), with a long slender muzzle, have been obtained. In a specimen[615] from the former locality, the skull, jaws with teeth, the entire vertebral column, and many parts of the skeleton are preserved: the entire length is but three feet.
[615] Now in the British Museum: see Petrif. p. 178, where a detailed description of this unique fossil is given. See also Ossem. Foss. tom. v. pl. vi.
Streptospondylus, Lign. 206, figs. 5, 7.--Baron Cuvier in his celebrated work, "Recherches sur les Ossemens Fossiles," has given an elaborate description of the remains of two kinds of slender-nosed Crocodilians, from the Kimmeridge clay of Honfleur, and the Oxford clay of Havre.
The specimens consist of the jaws with teeth, vertebræ, and some bones of the extremities.[616] In one species, the vertebræ are biconcave; in the other, they are convexo-concave, and present a remarkable deviation from the recent Crocodilian type, namely, that they are placed in a reversed position,--the convex face of the vertebra being directed anteriorly, or towards the cranium, and the concavity posteriorly; the name of the genus, Streptospondylus (reversed-spine), denotes this peculiarity of structure. The bodies of three or four large convexo-concave cervical vertebræ, were discovered in the Tilgate strata many years since, and are described in my various works (Geol. S. E. p. 300); but no suspicion was then entertained of their belonging to this genus, although I had repeatedly compared them with the figures of the Honfleur crocodile,[617] the imperfect state of the processes obscuring their true characters. Professor Owen first detected the true character of these Wealden vertebræ, in a large cervical, six inches long (now in the British Museum), in which two oblique processes are preserved on the concave end of the bone, their flat, oblong, articular faces, are directed downwards and outwards,--a character which at once proves them to be the posterior pair, for the anterior oblique processes would be directed upwards and inwards.[618] Vertebræ of the same species occur in the Wealden of the Isle of Wight; and of another species in the Oolite at Chipping Norton, and in the Lias of Whitby.
[616] Oss. Foss. tom. v. p. 143, pl. viii. ix.
[617] Reduced figures of two of these vertebræ are given in _Lign. 206_, figs. 5 and 7, p. 653.
[618] Brit. Assoc. Rep. 1841, p. 92. The position of the articulating surfaces of the oblique processes (zygapophyses) in more perfect specimens, subsequently discovered in various localities of the Wealden, established the existence not only of a saurian allied to the Streptospondylus, but also of other reptiles whose spinal column was wholly or in part made up of vertebræ which were convex in front and concave behind, as in the cervicals and anterior dorsals of certain mammalia. Some of these fossil vertebræ Dr. Melville has referred, with great probability, to the cervical region of the Iguanodon (see Petrif. p. 259); others cannot at present be satisfactorily assigned to any known genus of reptiles.
A concavo-convex caudal vertebra, with the relations of which I am unacquainted, was found in the same quarry in Tilgate Forest; a reduced outline of this unique fossil is given in Lign. 206, fig. 1. The centrum is of a sub-cylindrical form, and the articular face in front is concave, and that behind, convex; with a chevron-bone that is anchylosed to the body of the vertebra, as in some of the caudals of the Mosasaurus, and terminates in an inferior spine (f.); the pair of anterior oblique processes remains; the neural spinous process is destroyed.
Cetiosaurus.--From a considerable number of vertebæ and bones of the extremities of some gigantic aquatic reptiles, discovered in the Oolite in various places in Oxfordshire, Northamptonshire, and Yorkshire, the present genus was established; the name being intended to indicate a distant general resemblance of these extinct Saurians to the Cetaceans.[619] The vertebra; differ from those of the Iguanodon in having their articular faces of a sub-circular form, and the body relatively short; the anterior face is nearly flat, and the posterior concave, in the dorsal vertebra;; but in the caudal both faces are concave, and have a well-defined elevated margin, which gives the body a deeply excavated character, easily recognizable. Vertebræ of this kind were among my earliest discoveries in the strata of Tilgate Forest. (Geol. S. E. p. 282.) Some specimens are eight inches in the transverse diameter of the articular face, and but four and a half inches in the antero-posterior length of the body.[620] The original animals are supposed to have been of aquatic, and probably of marine habits, on the evidence of the sub-biconcave structure of the vertebræ and of the coarse cancellous tissue of the long bones, which are destitute of a medullary cavity. They must have rivalled the modern whales in bulk, for some specimens indicate a length of forty or fifty feet; they are supposed to have had web-feet, and a broad vertical tail.[621]
[619] Proc. Geol. Soc. vol. iii. p. 457.
[620] The osteological characters of these remains, and the physiological relations of the original animals, are described in Brit. Assoc. Rep. 1841, pp. 94-102.
[621] Brit. Assoc. Rep. 1841, p. 102.
Polyptychodon.[622]--The remains of another gigantic marine Saurian have been discovered in the Green Sand at Hythe, in Kent; they consist chiefly of the bones of the pelvis and hinder extremities.[623] The femur must have been nearly four feet in length. The long bones have a cancellated structure, without a medullary cavity 3 the outer surface is finely striated. Probably in a recent state the cells were filled with oil, as in the Cetacea. Neither the vertebræ nor the teeth of this reptile are known; but, provisionally, these remains have been referred to the same animal as that to which the large, conical, longitudinally ridged teeth belonged, which have been found in the Kentish Rag, at Maidstone, and in the Chalk of Sussex, and have been named Polyptychodon continuus.[624] Teeth of P. interruptus are not unfrequent in the Cretaceous series; and a portion of the lower jaw of this species, from the Chalk of Kent, is in Mr. Toulmin Smith’s collection.[625]
[622] Poly-ptych-odon; many-wrinkle-tooth.Return to passage
[623] Geol. Proc. vol. iii. p. 449. The bones from Hythe were presented by their discoverer, H. B. Makeson, Esq., to the British Museum. See Petrifactions, p. 200.Return to passage
[624] Owen’s Monograph, Cretac. Reptilia, Pal. Soc. p. 47.Return to passage
[625] Ibid. p. 55. See also Dixon’s Foss. Suss. p. 378. Teeth of the Polyptychodon are figured in Odontography, pl. lxxii.Return to passage