V. THE EXISTING ORDERS OF MAMMALS
PROTOTHERIA--MONOTREMATA
Apart from those creatures whose fragmentary remains have been considered in the last chapter, and which belong to the earliest of mammaliferous strata, the remains of Mammalia are all referable to existing orders. In the pages which follow we shall therefore deal with the actual representatives of living families side by side with their extinct relatives. The existing orders of Mammalia, together with those of their fossil allies, can be plainly divided into two great subdivisions, or, as we shall term them, sub-classes; the Mammalia as a whole being termed a class of the Vertebrata comparable with the class Reptilia, etc. It has been usual, owing to the initiative of Professor Huxley, to divide the Mammalia into three divisions of primary importance. We shall adduce reasons later for not accepting this mode of division, but that which allows of only two primary divisions. These two divisions are (1) Prototheria and (2) Eutheria. Whether the Multituberculata, Trituberculata, and Triconodonta, considered in the last chapter, are really to be distributed among these two sub-classes is a matter upon which it is possible to form an opinion, but not to dogmatise. The Prototheria stand at the base of the mammalian series, and present many likenesses to the Sauropsida; the Eutheria are the animals which are most fully differentiated as mammals. We shall commence with
SUB-CLASS I.--PROTOTHERIA.
To this group belongs the order Monotremata, and possibly also the so-called Allotheria or Multituberculata. As, however, {106} the latter are only known from very fragmentary remains, which are not sufficient to determine the systematic position of the animals of which they are fragments, I have not thought it worth while to attempt a serious definition of the order Multituberculata. I have introduced a short account of the principal facts which are known concerning the creatures grouped together under this name into the historical sketch of the progress of mammalian life in Chapter IV. As to the Monotremata, there is no question that they are entitled to rank in a group equivalent to that including all other mammals of which we have sufficient knowledge to construct a classificatory scheme. There have been, indeed, naturalists, such as Meckel, who would altogether deny the mammalian rank of these creatures.
The Monotremata or Ornithodelphia may be thus defined:--
Mammalia with no teats, but with a temporary pouch in which the young are hatched, or to which they are transferred after hatching, and into which open the ducts of the mammary glands. An anterior abdominal vein, or at least the membrane supporting it, persists throughout the abdominal cavity. Heart with an incomplete and largely fleshy right auriculo-ventricular valve. Brain without a corpus callosum. Shoulder girdle with a large coracoid reaching the sternum; clavicles and an interclavicle present. There are "marsupial" or epipubic bones attached to the pelvis. Vertebrae with no epiphyses for the most part. Ribs with only capitulum and no tuberculum. Mammary glands of the sudoriparous and not the sebaceous type of epidermic gland.[52] Oviparous, with a large-yolked and meroblastic ovum, enclosed within a follicle of two rows of cells.
To call these animals Mammalia is of course an abuse of the meaning of that word in one sense, but it is not in another; since the pouch of these Monotremes is, as has been explained elsewhere (p. 16), the real equivalent of a teat, and not of the pouch of the Marsupials.
The most salient characteristic of this group of mammals in the estimation of their position in the vertebrate series is not so much the fact that they are oviparous as that the eggs are large-yolked, and develop therefore, so far as regards their early stages, after the fashion of the egg of a reptile. The laying {107} of eggs, or at least ovoviviparity, would follow from the structure of the egg, since the abundance of yolk would do away with the necessity for a placenta. That the eggs had this Saurian characteristic was first definitely made known by Professor Poulton[53] for Ornithorhynchus, and his results were confirmed later for Echidna.[54] The structure of the eggs has, however, already been dealt with on p. 72. The fact that these animals lay eggs appears to have been known for a very long time, though rediscovered so lately as 1884 by Mr. Caldwell.[55] In connexion with the structure of the ova, the ovaries themselves and the oviducts are built upon the Sauropsidan plan. In the male the testes retain the primitive abdominal position. The fact that the urinary and genital products escape by means of their ducts into a chamber which also receives the end of the alimentary tract is not a distinctive feature of this group, inasmuch as it is seen in the Marsupials, and also in certain low Eutheria, such as the Beaver and other Rodents, and a few Insectivores. As to external features, the Monotremata show certain archaic characters. The unspecialised arrangement of the mammary glands has already been described. These animals are plantigrade, if the term may be used also to describe the aquatic Ornithorhynchus. The ears are absolutely destitute of a conch. The remarkable spur upon the hind-legs furnished with a gland, which is more marked in the male, and indeed disappears in the female of Ornithorhynchus, is a structure which argues the specialised condition of these two modern representatives of what must have been a large order in the past.
{108} [Illustration]
FIG. 51.--Ventral view of skull of Echidna aculeata, and right half of mandible, ang, Angle of mandible; aud.oss, auditory ossicles; cond, condyle of mandible; cor, coronoid process; max, maxilla; oc.cond, occipital condyle; pal, palatine; p.max, premaxilla; pt, pterygoid; sq, squamosal; ty, tympanic ring. (After Parker and Haswell.)
The skeleton shows numerous ancient characteristics. In the skull there is no demarcation of the orbit from the temporal fossa, a feature widely found in archaic mammals. The tympanic remains as a slender ring, there being no auditory bulla formed either from this or from any other bone. The malleus and incus are large, and thus reminiscent of the quadrate and articular bone of reptiles. In the lower jaw the absence of a marked coronoid process, and the absence of a firm ossification at the meeting of the two rami, may be a primitive state of affairs. It must be remembered, however, that the Cetacea show the same characters, though it is possible that they too are developed from a low mammalian stock. In the vertebral column we find the typical mammalian seven cervicals; but those characteristically mammalian structures the epiphyses are totally absent in Echidna, and only to be seen in the tail-region in Ornithorhynchus. In having only the capitular head to the ribs, these mammals are evidently far removed from all other mammals, and are even more reptilian than the Theromorphous reptiles. The large clavicles and the interclavicle (Fig. 52, {109} p. 109) are characteristic of the group, and the latter bone is peculiar to the Monotremata among mammals. So, too, is the large coracoid. In the scapula there is a spine which coincides with the anterior border of that bone. The arrangement of the muscles in this region proves conclusively that this projection is the homologue of the spine and the acromion of other mammals. Here, again, we have a point of likeness to the Cetacea.[56] In the pelvis the acetabulum is perforate (in Echidna), as in Sauropsida.
FIG. 52.--Side view of right half of the shoulder girdle of a young Echidna (Echidna aculeata). x 1. a, Acromion; c, coracoid; cb, coracoid border; cl, clavicle; css, coraco-scapular suture; ec, epicoracoid; gb, glenoid border; gc, glenoid cavity; ic, interclavicle; pf, postscapular fossa; ps, presternum; s, spine; ss, suprascapular epiphysis; ssf, subscapular fossa. (From Flower's Osteology.)
Considering the numerous very archaic features which the general structure of this group displays, it is surprising to find how typically mammalian they are in certain other peculiarities. The mammalian diaphragm, one of the distinguishing features of the class, is perfectly normal in the Monotremata. The alimentary canal shows no great divergences from the normal structure. The stomach is almost globular, with a projecting pyloric region in Ornithorhynchus; the intestine is divided into a "small" and "large" intestine by a slender caecum. The liver has the subdivisions that this organ usually shows in the Mammalia. However, the presence of the ventral mesentery and of the abdominal vein in Echidna and Ornithorhynchus has already been mentioned as a distinctive character. The peculiar and apparently partly primitive valve of the right ventricle has been described above (see p. 66). The brain is in most respects mammalian in its characters, but naturally shows some important differences. Dr. Elliot Smith, who has most recently studied this question,[57] is of opinion that the size of the cerebral hemispheres is not at all reptilian; indeed, it "greatly exceeds that of {110} many other mammals." In Echidna, too, but not in Ornithorhynchus, the hemispheres are well convoluted, though the arrangement of these convolutions cannot be brought into line with what is known concerning the convolutions upon the hemispheres of other mammals. It had been stated that in these animals, at least in Echidna, there were only two optic lobes, as in lower vertebrates, instead of the mammalian four. The late Sir W. H. Flower set this matter at rest,[58] and showed that Echidna was in this respect typically mammalian. The absence of the corpus callosum is one of the principal features separating the Monotremes from other mammals.
The Monotremata are represented to-day by two types, Ornithorhynchus and Echidna, which are no doubt worthy of being placed in separate families. Fossil remains of the group (apart from the problematical Multituberculata) are only known from Pleistocene times in Australia, and consist of the bones of a large species of Echidna, and some fragments of Ornithorhynchus, indicating a smaller animal than the living Platypus.
FIG. 53.--Brain of Echidna aculeata, dorsal view. (Nat. size.) (From Parker and Haswell's Zoology.)
FAM. 1. ECHIDNIDAE.--This family contains two genera, of which Echidna is the older and much the better known. The skin is abundantly covered with spines, with which are mingled hairs. The snout is tapering, the tail rudimentary, and the fingers and toes five in number. The spur and gland upon the calcaneum are smaller than in Ornithorhynchus. The claws are very strong, serving to tear open the ants' nests, upon the inhabitants of which the Echidna feeds, licking them up with a long extensile tongue like that of Myrmecophaga. In relation to this habit the salivary glands are enormously developed, and indeed the animal has been confounded with Myrmecophaga,[59] as the vernacular name "Australian Anteater" exemplifies. {111}
In the skull the Echidna differs from Ornithorhynchus in the greater extension backwards of the palatines, and the larger size of the pterygoids. The extent and relations of these bones to each other is not at all unlike that which obtains in many Whales. The premaxillae show traces of the same divergence followed by convergence of their ends that is seen in the Platypus. There are only sixteen pairs of ribs, and either three or four lumbar vertebrae. Echidna has no trace of teeth, and there are no horny pads which take their place; the mouth is as edentulous as in the true American Anteaters. The brain (Fig. 53) is marked by sulci, contrary to what we find in Ornithorhynchus. The genus has been divided into three species, but it is doubtful whether more than one can be allowed, which ranges from Australia through the Papuan region. While there is but one species of true Echidna, a New Guinea species must clearly be referred to a distinct genus Proechidna.[60] This animal is to be distinguished by the fact that there are usually but three toes on each foot. But there are copious rudiments of the other phalanges, upon which claws are sometimes developed. The beak is curved downwards, and the back is rather arched; the whole animal has the most singular likeness to an Elephant! The ribs are increased by one pair, and there are four lumbar vertebrae. The one species is named P. bruijnii. The Hon. W. Rothschild[61] distinguishes a form P. nigroaculeata, which is allowed by Mr. Lydekker.
FIG. 54.--Australian Anteater. Echidna aculeata. x 1/6.
{112}
The Echidna feeds like anteaters, by thrusting its tongue into an ant-hill, and waiting until it is covered with indignant and marauding ants, which are then swallowed. But this animal also devours worms and insects, which are extracted from their hiding-places by the tongue. It is mainly nocturnal, and prefers the seclusion of the densest scrubs of the bush, or rocky spots where it is free from intrusion. Dr. Semon did not find that the spur of this animal was used at all in self-defence; but he thinks that possibly the weapon may be used, in the breeding season only, in the combats of the males for the females, when perhaps, as has been shown to be the case in Ornithorhynchus, the gland attached to it produces a poisonous secretion.
The egg, as it appears, is transferred to the pouch by the mouth of the mother; the shell is broken by the emerging young one, which has an egg-breaking tubercle on its snout for this purpose; the mother removes the shell. When the young has attained a certain size, the mother removes it from the pouch, but takes it in from time to time to suckle it. When on her nightly rambles the young one is left in a burrow dug for the purpose. Dr. Semon was able, from his own observations, to substantiate this act of intelligence on the part of the Echidna. It is well known that the temperature of the Monotremes is less than that of higher mammals; in addition to this fact Dr. Semon found that the range of variation of temperature in the Echidna was as much as 13 degrees or more. It is thus intermediate between the "poikilothermal" reptiles and the "homoeothermal" mammals.
FAM. 2. ORNITHORHYNCHIDAE.--There is no need to attempt to define this family, since it contains but one genus Ornithorhynchus, with but one species, O. anatinus. The general aspect of the animal is well known. It is covered with dense fur of a blackish brown colour; the limbs are short and five-toed, the toes being webbed. The tail is longish and broad, being flattened from above downwards. The webbing on the anterior toes considerably outdistances the tips of the claws, as in the Seals. But this is not the case with the hind-feet. The "beak," which is broad and flat, and does actually suggest that of a duck, is not covered with horn, as is often stated, but with a fine, soft, sensitive, naked skin, which abounds in sense-organs of a tactile nature. As to characters derived from the skeleton, Ornithorhynchus has {113} seventeen pairs of ribs and only two lumbar vertebrae. The skull is expanded in front, and the bill is supported by two, at first diverging, and then converging, premaxillae. Between them is the famous "dumb-bell shaped bone," which is believed to be the representative of the reptilian prevomer. The pterygoids are smaller than in Echidna, and the hard palate does not extend so far back as in that genus. The brain of this genus is smooth.
FIG. 55.--Duck-billed Platypus. Ornithorhynchus anatinus. x 1/6.
The discovery of the real teeth of Ornithorhynchus only dates from the year 1888, when they were found by Professor Poulton[62] in an embryo. Later Mr. Thomas found[63] that the teeth persist for a considerable portion of the animal's life, and are only shed, like milk teeth, "after being worn down by friction with food and sand." We have already (p. 98) called attention to the general similarity of these teeth to those of certain of the earliest Mammalia and of mammal-like reptiles. The teeth are all molars, and they are either eight or ten in number. They are replaced by the horny plates of the adult animal; but the mode of replacement is curious. The plates are developed from the epithelium of the mouth, but round and under the true teeth; the epithelium of the mouth grows gradually under the calcified teeth, a method of growth which has possibly something to do with the shedding of the latter. The hollows and {114} grooves in the plates are the remains of the original alveoli of the teeth.
FIG. 56.--Skeleton of male Ornithorhynchus. Ventral view. The right fore-limb has been separated and turned round so as to bring into view the dorsal surface of the manus. The lower jaw is removed. acc.tars, Accessory tarsal bone supporting the spur; ant.pal.for, anterior palatine foramen; ast, astragalus; atl, atlas; ax, axis; bs.oc, basi-occipital; bs.sph, basi-sphenoid; calc, calcaneum; cbd, cuboid; cerv.rb, cervical rib; clac, clavicle; cond.for, foramen above inner condyle of humerus; cor, coracoid; cun, cuneiform of carpus; dent, horny dental plate; ect.cun, ecto-cuneiform; ent.cun, ento-cuneiform; ep.co, epicoracoid; epist, episternum; ep.pb, epipubis; fb, fibula; fem, femur; for.mag, foramen magnum; glen, glenoid cavity of shoulder-joint; glen, glenoid cavity for mandible; hum, humerus; in.cond, inner condyle of humerus; inf.orb.for, points to position of infra-orbital foramen; infr.proc, inferior processes of caudal vertebrae; int.rbs, intermediate ribs; isch, ischium; mag, magnum of carpus; max, maxilla; max.for, maxillary foramen; metat.I, first metatarsal; metat.V, fifth metatarsal; nas.cart, nasal cartilage; obt, obturator foramen; ol, olecranon; out.cond, outer condyle of humerus; pal, palatine; pat, patella; post.pal.for, posterior palatine foramen; pr.max, premaxilla; pr.st, presternum; pter, pterygoid; pub, pubis; rad, radius; scap, scapula; scaph, scaphoid of tarsus; scaph.lun, scapho-lunar; ses, sesamoid bones of wrist and ankle; sp, tarsal horny spur; sq, squamosal; tib, tibia; trd, trapezoid; trm, trapezium; tym.c, tympanic cavity; uln, ulna; unc, unciform; vom, vomer; x, dumb-bell shaped bone; zyg, zygomatic arch; I-V, digits of manus; V, foramen for fifth nerve. (From Parker's Zoology.)
{115}
The Duck-billed Platypus is, as every one knows, an aquatic animal. It is not found all over Australia, but is limited to the southern and eastern parts of that continent, and to Tasmania. The animal excavates a burrow for itself in the bank of the slow streams which it frequents. The burrow has one opening below the water and one above; and it is of some length, twenty to fifty feet. The Platypus feeds upon animal food, chiefly "grubs, worms, snails, and, most of all, mussels." These it stows away when captured into its capacious cheek-pouches. The food is then chewed and swallowed above the surface as the animal drifts slowly along. Dr. Semon, from whose work, In the Australian Bush, this account of the animal's habits is quoted, thinks that in the nature of the food of the creature the explanation of the loss of the teeth is to be found. He is of opinion that for cracking the hard shells of the mollusc Corbicula nepeanensis, upon which Ornithorhynchus mainly feeds, the horny plates are preferable to brittle teeth. Ornithorhynchus is apparently not eaten by the natives by reason of its ancient and fish-like smell. Besides, it is hard to catch on account of its diving capacities, which are aided by an acute sense of sight and of hearing. When the Duck-bill was first brought to this country it was believed to be a deliberate fraud, analogous to the mermaids produced by neatly stitching together the forepart of a monkey and the tail of a salmon.
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VI. INTRODUCTION TO THE SUB-CLASS EUTHERIA
SUB-CLASS II.--EUTHERIA
DEFINITION.--Mammalia with teats. Mammary glands of sebaceous type. Heart with entirely membranous and complete right auriculo-ventricular valve. Brain generally with a corpus callosum. Coracoid much reduced and not reaching sternum. No interclavicle. Vertebrae with epiphyses. Ribs double-headed. Viviparous, with a small ovum.
In this group are included not only the Eutheria in the sense of Huxley, but also his Metatheria. Though the Metatheria, or Marsupials as we shall term them, undoubtedly form a most distinct order of mammals, perhaps even a trifle more distinct than most others, their differences from the remaining tribes are not by any means so great as those which separate Ornithorhynchus and Echidna from all other mammals. In his well-known memoir upon the arrangement of the Mammalia,[64] Professor Huxley enumerated eleven characters as distinguishing the Metatheria either from the Prototheria or from the Eutheria. Of these only three were characters in which they approach the lower mammals. According to his showing, therefore, the preponderance of marsupial features are Eutherian. The three characters of Prototherian type are (1) the presence of epipubes; (2) the small corpus callosum; (3) the absence of an allantoic placenta.
The last of these can be dismissed, in consequence of the recent discovery of an allantoic placenta in Perameles. The first character is apparently a valid distinction between the Marsupials {117} and their mammalian relatives higher in the series; but it is not a character that should have been made use of by Huxley, since he believed in the existence of a corresponding element in the Dog. As to the corpus callosum (Fig. 50, p. 77) being small, that seems to be not more than a slight difference of degree.[65] A number of other characters of secondary importance were added by Huxley to the weight of evidence which led him to form a group Metatheria for the Marsupials. Some of these, however, are now known to be not evidence in that direction. For instance he observed that no Marsupial had more than a single successional tooth. It seems at the present moment to be fairly clear that Marsupials have a milk dentition like other Eutherians, but that only one of these teeth, the fourth premolar, comes to functional maturity. That it is really one of a complete milk series is evidenced by the fact that this tooth is differentiated contemporaneously with another series formerly held to belong to the so-called prelacteal dentition.[66] There still remains, of course, the actual fact that the milk dentition is not for the most part functional, but its significance breaks down with these fresh discoveries. Of this Professor Osborn has remarked: "The discovery of the complete double series seems to have removed the last straw from the theory of the marsupial ancestry of the Placentals." But Huxley did not lay much stress upon this matter of the teeth, since he observed that similar suppressions of the milk dentition were to be found in many other mammals admittedly Eutherian.
FIG. 57.--Brain of Echidna aculeata; sagittal section. ant.com, Anterior commissure; cbl, cerebellum; c.mam, corpus mammillare; col.forn, column of the fornix; c.qu, corpora quadrigemina; gang.hab, ganglion habenulare; hip.com, hippocampal commissure; med, medulla oblongata; mid.com, middle commissure; olf, olfactory lobe; opt, optic chiasma; tub.olf, tuberculum olfactorium; vent. 3, third ventricle. (From Parker and Haswell's Zoology.)
Huxley regarded the peculiarities in the reproductive organs {118} of the Marsupials as "singularly specialised characters," in no way intermediate in character. This view applies also to the pouch, which, as already stated, distinguishes the adults of that group. But the impossibility of using this last character as one of any importance has been shown by the discovery of rudiments of it in embryos of undoubtedly Eutherian mammals (see p. 18).
FIG. 58.--Sagittal section of brain of Rock Wallaby (Petrogale penicillata). ant.com, Anterior commissure; cbl, cerebellum; c.mam, corpus mammillare; c.qu, corpora quadrigemina; crur, crura cerebri; epi, epiphysis, with the posterior commissure immediately behind it; f.mon, position of foramen of Monro; hip.com, hippocampal commissure, consisting here of two layers continuous behind at the spleneium, somewhat divergent in front where the septum lucidum extends between them; hypo, hypophysis; med, medulla oblongata; mid.com, middle commissure; olf, olfactory lobe; opt, optic chiasma; vent. 3, third ventricle. (From Parker and Haswell's Zoology.)
Less stress is laid now upon the existence of four molars in the Marsupials as dividing them from the higher mammals than was formerly the case. The total dentition of the group is on the whole composed of more numerous individual teeth than in the typical Eutheria; but we have exceptions like the Whales, the Armadillo Priodontes, and the Manatee; or better, because free from the suspicion of secondary multiplication, Otocyon and occasionally (according to Mr. Thomas) Centetes. In the last two there are at least sometimes four molars.
On the other hand, a few archaic characters of some importance crop up here and there among the Marsupials, which are sometimes held to point to a primitive ancestry. It has been remarked that in Marsupials it is the fourth toe which is dominant in size, whereas in Ungulates it is the third. An attempt has been made to explain this on the view (reasonable enough in itself) of a tree-living ancestry for the group. A greater development of the fourth toe is, however, by no means a necessary character of arboreal creatures; the Primates themselves are an exception. Nor is this prevalence universal among the Marsupials; {119} in Myrmecobius (alone) is the third toe the longest; and no great difference can be detected between the third and fourth toes in the case of the genera Phascologale, Didelphys, and some others. Professor Leche compares the predominance of the fourth toe with the hyperphalangeal condition in the fourth toe of the embryo Crocodile, and considers it an archaic feature, not surpassed by the ancient characteristics of the Monotremata. Again it has been pointed out that in Phascologale and Perameles, the epistropheus (axis vertebra) has a separate rib as in Ornithorhynchus. In the third place, the likeness of the teeth of Myrmecobius to those of Ornithorhynchus is an argument in the same direction, which is furthermore supported by the great age (Mesozoic) of the Metatherian group, if we are right in regarding those extinct creatures as Marsupials.
We may now mention certain facts which are not so generally used. The partly primitive structure of the right auriculo-ventricular valve in the Monotremata has no counterpart in any Marsupial which has been dissected; but there are traces in the latter of the characteristic "ventral mesentery" of Ornithorhynchus and Echidna.[67] Mr. Caldwell's interesting observation upon the segmenting egg of the Marsupial, the incompleteness of the first segmentation furrow (reminding us of the meroblastic ovum of the Monotreme), may possibly not turn out to be so exclusively Marsupial a feature as has been thought.
The balance of evidence thus points to the nearer relationship of the Marsupials to the Eutherian mammals; and their great specialisation combined with certain evidences of degeneration (disappearance in part of the milk dentition), and their age, point to the fact that they are, at any rate, the descendants of an early form of Eutherian. But they must have separated from the Eutherian stock after it had acquired a definite diphyodonty and the allantoic placenta, the two principal features of the Eutherian as opposed to the Prototherian mammals.
Nevertheless it seems probable that the Marsupial tribe is derived from some of the earliest Eutherians. And on this view may be explained the retention of Prototherian characters.
The remaining Eutheria are obviously all to be referred to one great division with the possible exception of the Whales, whose affinities form one of the principal difficulties to the student {120} of this group. A short resume of what is at present thought of the systematic position of this anomalous order is appropriate here. Albrecht went so far as to regard the Cetacea as the nearest group of animals to the hypothetical Promammalia.[68] But discounting his arguments by the removal of such of them as relate to structure plainly altered by the singular mode of life of these creatures, there is really a great deal to be said in favour of his view.
The chief facts which argue a primitive position among mammals for the Cetacea are perhaps: (1) the slight union of the rami of the lower jaw; (2) the occasionally rather marked traces of the double constitution of the sternum; (3) the long and simple lungs; (4) the retention of the testes within the body-cavity; (5) the occasional presence (in Balaenoptera) of a separate supra-angular bone. These points, however, are but few, and are not of such great weight as those which ought to be present to establish a claim to separate treatment for the Cetacea as opposed to the Eutheria. If this group of mammals can be tacked on anywhere, it appears to us that the nearest relatives are not, as is sometimes put forward, the Ungulata or the Carnivora, but the Edentata. There are quite a number of rather striking features in which a likeness is shown between these apparently diverse orders of mammals. The chief ones are these: (1) the existence of traces of a hard exoskeleton, of which vestiges remain in the Porpoise; (2) the double articulation of the rib of the Balaenopterids to the sternum, with which compare the conditions obtaining in the Great Anteater; (3) the concrescence of some of the cervical vertebrae; (4) the share which the pterygoids may take in the formation of the hard palate; (5) the fact that in the Porpoise, at any rate, as in many Edentates, the vena cava, instead of increasing in size as it approaches the liver, diminishes.
Another group which is perfectly isolated is that of the Sirenia. The alliance advocated by some with the Cetacea, and quite recently renewed by Professor Haeckel, is contradicted by so many important features that it seems necessary to abandon it. The recent discovery of a fossil Sirenian jaw by Dr. Lydekker with teeth highly suggestive of those of Artiodactyla, may prove a clue. A third group which is so isolated as to have been placed in a {121} primary division, proposed to be called Paratheria, is that of the Edentates. Probably the group so called should really be divided into the Edentata and the Effodientia, the latter containing the Old World forms. Whether or not it be ultimately shown that the Ganodonta are ancestral Edentates (sensu strictiori), the connexion of the group with others is not at present plain. The same is the case with the extensive order of Rodents. It is true that the extinct order of the Tillodontia shows certain Rodent-like characters on the one hand, and likenesses to Ungulates on the other. Certain likenesses shown by such apparently diverse animals as the Rabbit and the Elephant used to be insisted upon by Professor Huxley. For the present, however, the Rodents must remain as an isolated group with only very dubious affinities to others. The remaining groups of existing mammals are easier to connect. At first the differences between a Cat and a Horse seem to be quite as wide as those which separate any two of the higher Eutherian orders. But it seems to become clearer and clearer, as palaeontological investigation proceeds, that the bulk of the Ungulate and the Carnivorous, Insectivorous, and perhaps Lemuroid stocks converge into the early Eocene Creodonta. From the Lemuroid branch the higher Primates can be derived. The only "Ungulates" which cannot be fitted in with some reasonable probability is the group of the Proboscidea. But of the early forms of this division we have at present no knowledge.
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VII. EUTHERIA--MARSUPIALIA
ORDER I. MARSUPIALIA[69]
The Marsupials may be thus defined:--Terrestrial, arboreal, or burrowing (rarely aquatic) mammals, with furry integuments; palate generally somewhat imperfectly ossified; jugal bone reaching as far as the glenoid cavity; angle of lower jaw nearly always inflected. The clavicle is developed. Arising from the pubes are well developed and ossified epipubic bones. Fourth toe usually the most pronounced. Teeth often exceed the typical Eutherian number of forty-four; molars generally four on each side of each jaw. As a rule but one tooth of the milk set is functional, which is (according to many) the fourth premolar. Teats lying within a pouch, in which the young are placed. Young born in an imperfect condition, and showing certain larval characters. There is a shallow cloaca. The testes are extra-abdominal, but hang in front of the penis. In the brain the cerebellum is completely exposed; the hemispheres are furrowed, but the corpus callosum is rudimentary. An allantoic placenta is rarely present.
Structurally the Marsupials are somewhat intermediate between the Prototheria and the more typical Eutheria, with a greater resemblance to the latter.
FIG. 59.--Rock Wallaby (Petrogale xanthopus), with young in pouch. x 1/7. (After Vogt and Specht.)
The name Marsupial indicates what is perhaps the most salient character of this order. The pouch in which the young are carried is almost universally present. It is less developed {123} on the whole in the Polyprotodont forms, such as the Thylacine, Dasyures, etc., but is found in so many of them that the two divisions of the Marsupials, the Diprotodonts and the Polyprotodonts, cannot be raised to distinct orders on this and other grounds. The marsupial pouch of the Marsupials must not, as has been already pointed out, be confounded with the pouch of the Monotreme mammals. Distinct teats are found in the marsupium of the Marsupials, while there are none in the mammary pouch of the Monotreme, the pouch itself indeed representing an undifferentiated teat, of which the walls have not closed up. The pouch opens forward in the Kangaroos, and backwards in the Phalangers and in the Polyprotodonts. Its walls are supported by a pair of bones diverging from each other in a [70]-shaped manner; these are cartilaginous and vestigial in the Thylacine. They {124} are the precise equivalents of similar bones in the Monotremata. It has been held, but apparently erroneously, that these bones are mere ossifications in the tendons of the external oblique muscle of the abdomen, or of the pyramidalis of the same region; and vestiges have been asserted to exist in the Dog. Such bonelets are undoubtedly present in the Dog; but it seems clear from their development in Marsupials, as structures actually continuous with the median unossified portion of the symphysis pubis, that the "marsupial bones" belong to that part of the skeleton, and that they correspond with the epipubis of certain amphibians and reptiles. The pouch, it may be remarked, exists in a rudimentary form in the males of many Marsupials.
FIG. 60.--Ventral surface of innominate bone of Kangaroo (Macropus major). x 1/3. a, Acetabulum; ab, acetabular border of ilium; is, iliac surface; m, "marsupial" bone; pb, pubic border; pt, pectineal tubercle; s, symphysis; si, supra-iliac border; ss, sacral surface; thf, thyroid foramen; ti, tuberosity of ischium. (From Flower's Osteology.)
FIG. 61.--Mammary foetus of Kangaroo attached to the teat. (Nat. size.) (From Parker and Haswell's Zoology.)
The most salient feature in the life-history of the Marsupials is the imperfect condition in which the young are born. The egg is no longer laid, as in the Monotremes; but curiously enough the ovum, which has the small size of that of the Eutheria, divides incompletely at the first division (as Mr. Caldwell has shown), and this developmental feature may perhaps be looked upon as a reminiscence of a former large-yolked condition. The young when born are small and nude; the newly born young of a large Kangaroo is perhaps as large as the little finger. The young are transferred by the lips of the mother to the pouch, where they are placed upon a teat. It is an interesting fact that they are not merely imperfect foetuses, but that they are actual larvae. They possess in fact at any rate one larval organ in the shape of {125} a special sucking mouth. This sucking mouth is an extra-uterine production, and is of course an adaptation to the particular needs of the young, just as are other larval organs, such as the chin-suckers of the tadpole, or the regular ciliated bands of the larvae of various marine invertebrate organisms.
There are a number of other features which distinguish the Marsupials from other mammals.
The cloaca of the Marsupials is somewhat reduced, but is still recognisable. Its margins in Tarsipes are even raised into a wall, which projects from the body.
The tooth series of the Marsupials was once held to consist of one dentition only, with the exception of the last premolar, which has a forerunner. The interpretation of the teeth of Marsupials are various. Perhaps most authorities regard the teeth as being of the milk dentition, with the exception of course of the single tooth that has an obvious forerunner. But there are some who hold that the teeth are of the permanent dentition. In any case it is proved that a set of rudimentary teeth are developed before those which persist. Those who believe in the persisting milk dentition describe these as prelacteal. Another matter of importance about the teeth of this order of mammals is that their numbers are sometimes in excess of the typical Eutherian 44. This, however, holds good of the Polyprotodonts only.
It was for a long time held that the Marsupials differed from all other mammals in having no allantoic placenta. But quite recently this supposed difference has been proved to be not universal by the discovery in Perameles of a true allantoic placenta. The Marsupials have been sometimes called the Didelphia. This is on account of the fact that the uterus and the vagina are double. Very frequently the two uteri fuse above, and from the point of junction an unpaired descending passage is formed (see Fig. 48 on p. 74).
A character of the brain of Marsupials has been the subject of some controversy. Sir Richard Owen stated many years ago that they were to be distinguished from the higher mammals by the absence of the corpus callosum. Later still it was urged that a true corpus callosum, though a small one, was present; while, finally, Professor Symington[71] seems to have shown that {126} the original statement of Owen was correct, at least in part. It is at most feebly developed (see Fig. 58, p. 118).
As to skeletal characters, the Marsupial skull has on the whole a tendency towards a permanent separation of bones usually firmly ankylosed. Thus the orbitosphenoids remain distinct from the presphenoid. The palate is largely fenestrated, a return as it were--says Professor Parker--to the Schizognathous palate of the bird. The mandible is inflected; this familiar character of the Marsupials goes back to the earliest representatives of the order in Mesozoic times (see p. 96); but it is not absolutely universal, being absent from the much weakened skull of Tarsipes. On the other hand, the inflection is nearly as great in certain Insectivores, in Otocyon, etc. The malar always extends back to form part of the glenoid cavity. The shoulder girdle has lost the large coracoid of Monotremes; this bone has the vestigial character that it possesses in other Eutheria. The clavicle is present except in the Peramelidae. A third trochanter upon the femur seems to be never present.
FIG. 62.--Skull of Rock Wallaby (Petrogale penicillata). (Ventral view.) ali, Alisphenoid; bas.oc, basi-occipital; bas.sph, basi-sphenoid; ex.oc, ex-occipital; ju, jugal; max, maxilla; pal, palatine; par.oc, paroccipital; p.max, premaxilla; pr.sph, presphenoid; pt, pterygoid; sq, squamosal; ty, tympanic. (From Parker and Haswell's Zoology.)
The Marsupials cannot be regarded as an intermediate stage in the origin of the Eutheria for a number of reasons. In the first place, the nature of their teeth shows them to be degenerate animals; one set, whether we regard it as the milk or permanent dentition, has become vestigial. The recent discovery of a true allantoic placenta in Perameles removes one reason for regarding {127} the Marsupials as primitive creatures. It implies on the whole that the Marsupials have sprung from a stock with an allantoic placenta. The alternative is to assume the independent development of an allantoic placenta in both groups of the Mammalia; unless indeed the genus Perameles is to be held to be the most primitive race of Marsupials living, a hypothesis which does not appear on the face of it likely. So long as it was believed that the mammary pouch of the Monotremes was the equivalent of the marsupium of the Marsupials, the persistence of this structure seemed to be a bond of union between the groups. But it is now known that the marsupium is a special organ confined to the Marsupials, an argument which is rather in favour of their being a lateral development of the mammalian stem. It is to be remarked also that the marsupium is feeblest in the Polyprotodonts, which may perhaps be looked upon as the most primitive of the Marsupials, owing to their more numerous teeth and other points to be referred to immediately.
Not only are the Marsupials interesting from the point of view of their structure; their present and past distribution is of equal interest. During the Mesozoic epoch they occurred in Europe and North America; but not, so far as negative evidence means anything, in Australia, which is now their headquarters. In Europe Marsupials lingered on into the Tertiary period, when they finally became extinct. In America, of course, the group has persisted to the present day. Now it is important to notice that the two main subdivisions of the Marsupials, the Polyprotodontia and the Diprotodontia, exist to-day in both Australia and South America. These two divisions, it should be explained, differ principally in that one has numerous, the other rarely more than two,[72] incisors in the lower jaw. It is perhaps the more widely distributed opinion that the Polyprotodontia are the more archaic group; this opinion rests upon one or two facts in addition to the absence of specialisation in the incisor teeth. Among the Polyprotodontia the total number of teeth is greater--a clearly primitive character; secondly, the general form of the body of these animals, with four subequal limbs and carnivorous or omnivorous diet, contrasts with the purely vegetarian and much specialised Kangaroos at any rate. Finally--and sufficient stress {128} has perhaps not been laid upon this matter--the brain among the Polyprotodonts is less convoluted than among the genera of the other division. This statement is of course made with due regard to parallelism in size (see p. 77). It is well known that the complexity of a brain bears a distinct relation to the size of its possessor within the group. Now the most ancient Marsupials are decidedly more Polyprotodont-like. No European form from the earlier periods is distinctly to be referred to the Diprotodonts. But both divisions now exist in America and Australia.
We must assume, therefore, one of three hypotheses. Either the differentiation into the two great divisions occurred in Jurassic or Cretaceous times before the migration of the order southwards; or the Diprotodont type is only a type, and not a natural group, i.e. it has been separately evolved in America and Australia; or, finally, there was formerly a land-connexion in the Antarctic hemisphere, along which the Diprotodonts of Australia wandered into South America. The middle hypothesis has this to commend it, that syndactylism occurs in both divisions, and that in some Diprotodonts the pouch opens backwards as it does in the Polyprotodonts. So great are the resemblances that but little difference is really left--of great importance that is to say. Hence it is not difficult to imagine the reduction of the incisors having taken place twice. In favour of the first hypothesis there are no positive facts. Finally, in favour of the last, which is so strongly supported by the facts of distribution derived from the study of other groups of animals,[73] there is at least this striking fact or rather series of facts: that some of the South American fossil Polyprotodonts have a "strictly Dasyurine relationship."[74] If there has not been a direct migration, then the Dasyurine type has been twice evolved, an improbability that few will attempt to explain away. In any case we shall adopt here the usual division of the Marsupials into Diprotodontia and Polyprotodontia.
SUB-ORDER 1. DIPROTODONTIA.
This group includes the herbivorous Marsupials. The incisors are as a rule three above, but one only in the Wombats. Below {129} is one strong pair, with occasionally one or two rudimentary incisors. The upper canines, if present, are not large. The molars are tuberculate or ridged. All Marsupials (except the Wombats) to some extent, and the Macropods especially, are characterised by the prolongation of the tubes of the dentine into the clear enamel. The significance of this fact is, however, lessened by the fact that the same penetration of the enamel by dentinal tubes occurs in the Jerboa, the Hyrax, and some Shrews. The feet have two syndactylous toes,[75] less marked in the Wombats than in the Kangaroos and Phalangers.
FIG. 63.--Skull of Wombat (Phascolomys wombat). (Lateral view.) ang, Angular process; cond, condyle of mandible; ext.aud, opening of bony auditory meatus; ex.oc, exoccipital; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; p.max, premaxilla; sq, squamosal; ty, tympanic. (From Parker and Haswell's Zoology.)
This order is mainly Australian at the present day, using the term of course in the "regional" sense (see p. 84); the only exception indeed to this statement is the occurrence of the genus Caenolestes in South America. But it is now known that Diprotodont Marsupials formerly existed in the same part of the world.
FIG. 64.--Bones of right foot of Kangaroo (Macropus bennetti). a, Astragalus; c, calcaneum; cb, cuboid; e^3, ento-cuneiform; n, navicular; II-V, second to fifth toes. (From Flower's Osteology.)
{130}
FIG. 65.--Skeleton of Wallaby (Macropus ualabatus). The scapula is raised somewhat higher than in nature. The end of the tail is omitted. The head of the femur has been separated from the acetabulum. acet, Acetabulum; acr, acromion process; ast, astragalus; cal, calcaneum; cbd, cuboid; chev, chevron-bones; cl, clavicle; cun, cuneiform of carpus; epi, epipubis; fb, fibula; fem, femur; hd, head of femur; hu, humerus; il, ilium; isch, ischium; obt, obturator-foramen; orb, orbit; pis, pisiform; pub, pubis; rad, radius; rb^1, first rib; rb^{13}, last rib; sc, scapula; st, sternum; tb, tibia; troch, great trochanter of femur; uln, ulna; unc, unciform; IV, fourth toe. (From Parker and Haswell's Zoology.)
FAM. 1. MACROPODIDAE.--This family contains the Kangaroos, Wallabies, Rat-Kangaroos, and Tree-Kangaroos. With the exception of Dendrolagus the family is terrestrial, and its numerous species progress by leaps effected by the long hind-limbs, which are decidedly, often greatly, longer than the fore-limbs. In the hind-limb the fourth toe is very long and strong; the fifth moderately so; the second and third are slender and united by skin. The tail is always long, but differs in its characters from {131} genus to genus. The stomach is much sacculated. The dental formula is I 3/1 C (1 or 0)/0 P 2/2 M 4/4. The atlas is often open below, forming thus an incomplete ring.
Though the number of the incisor teeth in the adult Diprotodonts is never more than three on each side in each jaw, more numerous rudiments are present. Mr. M. Woodward[76] has lately investigated the subject with interesting results. He finds that many species present decided traces of two additional incisors, raising the total to that which characterises the Polyprotodontia; but in two cases, viz. Macropus giganteus and Petrogale penicillata, a sixth is present, the total number being thus in excess of that found in any other Marsupial. This, as the author himself admits, proves too much. No mammal is known which in the adult condition has so many incisors; nor do the fossil Mammalia help us to get over the difficulty; even among reptiles it is not usual for so many teeth to occur upon the premaxillaries.
It is a curious fact that the two long lower incisors can be used after the fashion of a pair of scissors, or rather a pair of shears. Their inner edges are sharpened, and they are capable of some motion towards and away from each other; by their means grass is cropped.
The stomach of Macropus (and of other allied genera) is peculiar by reason of its long and sacculated character; the oesophagus enters it very near the cardiac end, which is bifid. Messrs. Schaefer and Williams[77] have shown that the squamous, non-glandular epithelium of the oesophagus extends over the greater part of the stomach, only the pyloric extremity and one of the two cardiac caeca being lined with columnar epithelium.
The Macropodidae are clearly divisible into three sub-families, which are distinguished by marked anatomical characters.
In the sub-family MACROPODINAE (including the genera Macropus, Petrogale, Lagorchestes, Dorcopsis, Dendrolagus, Onychogale, and Lagostrophus) there is no hallux, and the tail is hairy. The oesophagus enters the stomach near the cardiac end. The caecum when short has no longitudinal bands; the liver has a Spigelian lobe.
The second sub-family, POTOROINAE or HYPSIPRYMNINAE (including the genera Potorous, Aepyprymnus, Bettongia, and {132} Caloprymnus), consists of smaller animals than the Macropodinae, which, however, resemble them in having no hallux, but a hairy tail. The oesophagus enters the stomach near the pyloric end of that organ. The caecum, though short, has lateral longitudinal bands. The liver has no special Spigelian lobe. The canines are always present, being rarely so in Macropodinae, and are usually well developed.
The third sub-family, that of the HYPSIPRYMNODONTIDAE, is doubtfully referable to the family; it consists of but one genus Hypsiprymnodon, which is in many points more like a Phalanger than a Kangaroo. It has an opposable hallux and a non-hairy, but scaly, tail. It has canines in the upper jaw.
FIG. 66.--Red Kangaroo. Macropus rufus. x 1/18.
SUB-FAM. 1. MACROPODINAE.--The genus Macropus includes not only the Kangaroos but also the Wallabies, which are really indistinguishable, though they have sometimes been placed in a separate genus Halmaturus. The genus thus enlarged contains twenty-three species. It may be thus characterised: the ears are long, the rhinarium is usually naked, but in M. giganteus and others a band of hairs descends to the upper lip; a naked band extends from the ankle to the pads on the digits, which is interrupted in M. rufus by a band of hairs just in front of the digits. The mammae are four. The tail is not bushy, {133} but is crested in M. irma. They are for the most part found on the Australian continent, but some species are found in the islands to the north which belong to the Australian region. Thus M. brunii, which is of interest as the first Kangaroo seen by a European, is a native of the Aru islands. A specimen of this animal, which was then living in the garden of the Dutch governor of Batavia, was described by Bruyn in the year 1711. M. rufus, the largest member of the group, is remarkable for the red secretion which adorns the neck of the male. It is caused by particles which have the appearance and colour of carmine. M. giganteus is not, as its specific name might imply, the "giant" of the race; its dimensions are given as 5 feet, while M. rufus is said to attain a length of 5 feet 5 inches, exclusive (in both cases) of the tail.
The account which Sir Joseph Banks gives[78] in his diary of the Kangaroo is interesting, since he was one of the first naturalists to see that creature. In July 1770 it was reported to him that an "animal as large as a greyhound, of a mouse colour, and very swift" had been seen by his people. A little later he was surprised to observe that the animal "went only upon two legs, making vast bounds just as the jerboa does." The second lieutenant killed one of these Kangaroos, of which Sir Joseph Banks wrote that "to compare it to any European animal would be impossible, as it has not the least resemblance to any one I have seen. Its fore-limbs are extremely short and of no use to it in walking; its hind, again, as disproportionately long; with these it hops seven or eight feet at a time, in the same manner as the jerboa, to which animal indeed it bears much resemblance, except in size, this being in weight 38 lbs., and the jerboa no larger than a common rat." The beast was killed and eaten, and proved excellent meat. Sir Joseph Banks' observations upon the leaping of the Kangaroo are of interest, because it is often asserted that the tail is largely made use of as a third foot or as a support. Mr. Aflalo declares in the most positive way that after repeatedly examining the tracks upon soft sand immediately after the animal had passed, not the very faintest trace of the impression of the tail could be discovered. The leaps of a big Kangaroo seem to be somewhat greater than is recorded {134} by Banks. It is said that 15 or even 20 feet are covered at a bound, and in bound after bound. But in walking slowly it can be readily seen from an inspection of Kangaroos at the Zoological Society's Gardens that the animal does rest upon its tail, which with the hind-legs forms a tripod.
Petrogale with six species comes next to Macropus, and is indeed only to be differentiated from it by the thickly-haired and more slender tail, which is not used, as it is sometimes in the Kangaroos, as an extra hind-limb. The Rock-Kangaroos live among rocks, which they climb, and from which they leap; and the tail acts rather as a balancing pole. The most elaborate account of the anatomy of Petrogale known to me is by Mr. Parsons.[79] The dentition as given by Mr. Thomas is I 3/1 C 0/0 Pm 2/2 M 4/4--that of Macropus without the occasionally occurring canine of the upper jaw. The osteological characters which separate it from Macropus are quite insignificant. Mr. Parsons mentions a wormian bone, "os epilepticum," at the junction of the coronal and sagittal sutures. It was found to occur in two out of five skulls examined, and appears not to occur in other Kangaroos. The palatine foramina of Petrogale are so large that the posterior part of the bone is only a narrow thickened ridge. The small intestine of P. xanthopus is 102 inches long, the large intestine 44 inches. The caecum has a length of 6 inches, and is not sacculated, differing in this from the caecum of Macropus major. The best known species are P. xanthopus and P. penicillata. The genus is confined to Australia itself, and does not enter Tasmania.
Onychogale includes the so-called "Nail-tailed Wallabies," which have a thorn at the end of the tail, reminding one of the Lion and the Leopard, whose tails have a similar armature. The muffle is hairy. Three species are allowed by Mr. Thomas.
Lagorchestes has, like the last genus, the rhinarium, i.e. that part of the nose immediately surrounding the nostrils, hairy instead of smooth as in the Kangaroos proper. It is distinguished from Onychogale by the absence of the terminal callosity to the tail, which is rather short. The name Hare-Kangaroo is given to the members of this genus (three species) on account of their exceeding fleetness. This genus is limited to Australia itself. L. conspicillatus is said to present "a remarkable {135} resemblance to the English hare," and L. leporoides was so called by Gould on account of general appearance as well as face.
Dorcopsis has shorter hind-legs than Macropus, and a naked muffle. The ears are small. The structure of D. luctuosa has been studied by Garrod,[80] who pointed out the existence of four enlarged hair follicles on the neck near the mandibular symphysis. These are, however, represented in the next genus Dendrolagus, and occur also in Petrogale. The limbs are not so disproportionate as in Macropus, and the tail is naked at the tip.
Dorcopsis and the next genus to be described, Dendrolagus, differ from Macropus and its immediate allies, Petrogale and Lagorchestes, in a number of anatomical points. In the first place, the premolars are twice the size of those of Macropus, and they have a characteristic pattern not observable in the Kangaroos. This consists of a median ridge (the whole tooth being rather prismatic in shape), with lateral ridges at right angles to it. The upper canines are developed, but are minute.
The stomach is not quite like that of Macropus, though built upon a similar plan. The blind cardiac extremity is a single, not a double cul-de-sac; in this it is like that of Petrogale. The distribution of the squamous, white, oesophageal epithelium is very much like that of Dendrolagus. In both genera the orifice of the oesophagus into the stomach is guarded by two strong longitudinal folds, which run for some distance towards the pylorus. In Dendrolagus, at any rate, this tract is bordered on each side by glandular patches. In Dendrolagus, moreover, the squamous epithelium does not extend into the cardiac cul-de-sac. This latter is separated from the rest of the stomach by two slightly diverging folds, which are faintly represented in Petrogale and in Halmaturus. In the last two genera the folds surrounding the oesophageal orifice are but slightly represented; better in Halmaturus than in Petrogale. But there are not the patches of glands already referred to. The small intestine of Dorcopsis is 97 inches in length, the large being 32, i.e. proportionately long, as in Marsupials generally. The small caecum (2-1/2 inches) is not sacculated.
The spleen is Macropodine, being [81]-shaped or [82]-shaped. The differences between Dorcopsis and the evidently closely allied Dendrolagus will be further considered under the description of {136} the latter. Dorcopsis is confined to New Guinea, and contains three species, viz. D. muelleri, D. luctuosa, and D. macleani. D. muelleri has a striking resemblance to Macropus brunii, with which it has been confounded. Though intermediate between Macropus and Dendrolagus, these Kangaroos are not arboreal.
The genus Dendrolagus is remarkable for its un-kangaroo-like habit of living in trees. In accordance with this change of habit is a relative shortening of the hind-limbs, a feature which begins to be observable in Dorcopsis. "The general build," writes Mr. Thomas, "is of the ordinary mammalian proportions, not macropodiform at all." The muffle is not naked for the greater part, though the shortness of the hairs gives that effect. As in Dorcopsis, but not as in Macropus, the bulla tympani is not swollen. There are altogether five species, the fifth, D. bennetti, having been lately described from specimens living in the Zoological Society's Gardens.
FIG. 67.--Tree-Kangaroo. Dendrolagus bennetti. x 1/12.
The anatomy of this genus has been described by Owen for D. inustus,[83] and by myself for D. bennetti. The stomach, which {137} has a single, not bifid, cul-de-sac, is sacculated by two principal bands and other subsidiary ones. Its internal structure has already been to some extent described. The spleen of D. bennetti is remarkable for the fact that it is not [84]-shaped, whereas D. inustus agrees with other Macropodines in the form of this organ. The small intestine of D. bennetti is 95 inches long, the large 38. The caecum appears to differ in the two species; it is smaller in D. bennetti, where it is only 2 inches in length. The most remarkable feature of the liver is the large size of the left lateral lobe and the bilobed condition of the Spigelian lobe; this at least was the case with D. bennetti. A recently-described species[85] has been attentively studied in its native haunts by Dr. Lumholtz.[86] It lives in the highest parts of the mountainous scrubs of Queensland, where it moves quickly on the ground as well as among the trees. It is hunted with Dingos by the "blacks," and is eaten by them.[87]
Lagostrophus is a generic name that has been proposed by Mr. Thomas for a small Wallaby 18 inches in length, which is distinguished by the fact that the long claws of the hind-limbs are entirely hidden by long and bristly hairs; the muffle is naked; there is no canine. The bullae are swollen. There is but one species of the genus, L. fasciatus, a native of West Australia.
SUB-FAM. 2. POTOROINAE.--Aepyprymnus and the other genera placed in this sub-family are known by the vernacular name of Rat-Kangaroos, or sometimes Kangaroo-Rats. The latter term has been called "incorrect," though it is just as good as the former, both of them in fact being inaccurate as implying some likeness to or relation with a Rat. The present genus has a partially hairy rhinarium; the auditory bullae are not swollen. It contains but one species, Ae. rufescens, a native of Eastern Australia, which is distinguished by its very long hind-feet.
Bettongia has long hind-feet as in Aepyprymnus, but the rhinarium is entirely naked instead of being partially hairy, while the ears are much shorter. The genus, which contains four species, is remarkable as being the only ground-living mammal with a prehensile tail, which it uses to carry grass, etc. {138} B. lesueuri burrows in the ground, often to so great a depth as 10 feet. The genus occurs in Tasmania as well as in Australia.
Caloprymnus, with one species, is a genus instituted by Mr. Thomas in his Catalogue of Marsupials for a form (C. campestris) which combines in a remarkable way the characters of Aepyprymnus, Bettongia, and Potorous. The external characters and the general shape of the skull are as in Bettongia, while the molars have the structure of those of Aepyprymnus. The last premolar is as in Potorous.
Of the genus Potorous there are three species, which are Tasmanian as well as Australian. Unlike the other Rat-Kangaroos, the hind-feet are comparatively short, and the animal is therefore less addicted to jumping than its relatives. The rhinarium is naked, and the ears are of fair length.
SUB-FAM. 3. HYPSIPRYMNODONTINAE.--The Musk-Kangaroo, Hypsiprymnodon, is the last genus of the present family, and the only genus of this sub-family. It is intermediate between the Macropodidae and the Phalangeridae, the annectant character being mainly the hind-feet, which though they have the same long fourth digit as the Kangaroos, have it more feebly developed, and possess also an opposable hallux, which is one of the salient features in the structure of the Phalangeridae. The tail is naked and scaly; the rhinarium is entirely naked. The ears are large and not furry. The single species, H. moschatus, appears to feed upon insects as well as vegetables.
"Its habits are chiefly diurnal, and its actions when not disturbed by no means ungraceful. It progresses in much the same manner as the Kangaroo-Rats (Potorous), to which it is closely allied, but procures its food by turning over the debris in the scrubs in search of insects, worms, and tuberous roots, frequently eating the palm berries, which it holds in its fore-paws after the manner of the Phalangers, sitting up on its haunches, or sometimes digging like the bandicoots." This is Mr. Ramsay's description of the animal, which he was the first to discover.[88]
FAM. 2. PHALANGERIDAE.--The genus Hypsiprymnodon bridges over the not very wide gap which separates the Kangaroos from the Phalangers. The Phalangers are Marsupials with five fingers and toes; the second and third toes are bound together by a {139} common integument as in the Macropodidae. The hallux is opposable and nailless. The tail is nearly always long and prehensile. The pouch is well developed; the stomach not sacculated; a caecum is present (except in Tarsipes). These are really the principal distinctions between the two families. In addition, it may be mentioned that the lower incisors have not a scissor-like action as in the Kangaroos.
The Phalangers may be divided into four sub-families.
The first of these, that of the PHALANGERINAE, contains the genera Phalanger (including Cuscus), Acrobates, Distaechurus, Dromicia, Gymnobelideus, Petaurus, Petauroides, Dactylopsila, Pseudochirus, and Trichosurus.
These genera agree in the following generalities:--Tail well developed, often very long; three incisors above, and at least two premolars both above and below; caecum long and simple; stomach without a cardiac gland; liver not very complicated by secondary furrows, with a distinct caudate lobe; the vaginal median culs-de-sac often coalesced; lungs with an azygos lobe.
FIG. 68.--Bones of leg and foot of Phalanger. ast, Astragalus; calc, calcaneum; cub, cuboid; ect.cun, ecto-cuneiform; ent.cun, ento-cuneiform; fb, fibula; mes.cun, meso-cuneiform; nav, navicular; tib, tibia; I-V, first to fifth toes. (After Owen.)
The second sub-family, PHASCOLARCTINAE (with the Koala only), is thus characterised:--Tail rudimentary; cheek-pouches present; superior incisors three, but only one premolar above and below; {140} caecum extraordinarily long; stomach with a cardiac gland; liver complicated by additional furrows, without a free caudate lobe; no azygos lobe to lungs; vaginal culs-de-sac free.
The third sub-family, PHASCOLOMYINAE, contrasts with the others as follows:--Tail rudimentary; cheek-pouches present, but rudimentary; one incisor on each side above, but no additional premolars; all the teeth rootless; caecum not peculiar in shape; stomach with a cardiac gland; liver complicated by secondary furrows, without a free caudate lobe; lung with an azygos lobe; vaginal culs-de-sac free.
The last sub-family, TARSIPEDINAE, is thus defined:--Tail long; tongue extensile; only one premolar; molars reduced; caecum absent.
FIG. 69.--Vulpine Phalanger. Trichosurus vulpecula. x 1/6.
SUB-FAM. 1. PHALANGERINAE.--The genus Phalanger embraces five species, sometimes called by the generic name of Cuscus. They are largish animals with short ears; only the end of the tail is naked. Of these animals only one species is found in Australia itself, the rest inhabiting the islands lying to the north. The Spotted Cuscus, Ph. maculatus, is in spite of its vegetarian diet, and perhaps on account of its spots, spoken of as the "Tiger Cat." Mr. Aflalo remarks of it that though provided with a prehensile tail, it is little better as a climber than the tailless Koala.
Trichosurus, including the "True Phalangers," includes largish species, which can be distinguished from the last genus by a chest-gland similar to that which occurs in Myrmecobius and some other Marsupials of the present group. There are but two species, which are purely Australian. The "Brush-tailed Opossum," T. vulpecula (perhaps better known as Phalangista {141} vulpina), like its American pseudo-namesake (a true Opossum, genus Didelphys), "plays 'possum" on occasions. The dental formula is I 3/2 C 1/0 Pm 2/3 M 4/4. The ears are shortish.
The Ring-tailed Phalangers, Pseudochirus, are more widely distributed than the last two genera; they range from Tasmania in the south to New Guinea in the north. They are not, however, ring-tailed, though the tip of the tail is generally white. As in the last genera, which have prehensile tails, the end of this appendage is naked. The mammae are four. The tooth formula is I 3/2 C 1/0 Pm 3/3 M 4/4. There are some ten species of the genus.
The Striped Phalanger, Dactylopsila trivirgata, is an animal about a foot long, whose identity can be ascertained by its striped, black and white skin. It is an arboreal creature that lives apparently both on leaves and grubs like so many arboreal creatures of quite different groups--Squirrels, for instance, and New-World Monkeys. The tooth formula is I 3/3 C 1/6 Pm 3/2 M 4/4.
Gymnobelideus leadbeateri is a small creature with a body 6 inches in length. It is restricted to the colony of Victoria. The general look is that of Petaurus; the ears are naked.
Dromicia is a genus of Phalangers which although devoid of a parachute, such as is possessed by certain genera that will be considered immediately, is able to leap with great agility from branch to branch. The ears are large and thin and almost naked; the tooth formula is I 3/2 C 1/0 Pm 3/3 M 4/4. They are minute creatures, the longest measuring, with the tail, but 10 inches. Dormouse-Phalanger is a name sometimes given to them. There are four species, ranging from Tasmania to New Guinea. The name Dormouse as applied to the genus seems to be owing to the way in which they hold a nut in the paws when feeding. D. nana is 4 inches long, with a tail of nearly the same length. It is thick at the base.
Distaechurus is the last genus of non-flying Phalangers. Its name refers to the arrangement of the hairs on the tail, which are disposed on either side in a row like the vane of a feather. The tooth formula is I 3/2 C 1/0 Pm 3/2 M 3/3, very nearly as in Acrobates. The ears are as in that genus.
Petaurus is the first genus of the Flying Phalangers, all of which are provided with a parachute-like expansion of the skin between the fore- and hind-limbs; the ears are large and naked; and the tooth formula is I 3/2 C 1/0 Pm 3/3 M 4/4. There are three {142} species of the genus, which extend through pretty well the entire Australian region. The term "flying" as applied to these and the other "flying" genera is of course an exaggeration. The animals cannot fly upwards; they can only descend in a skimming fashion, the folds of skin breaking their fall. P. breviceps is perhaps the best-known species. The body is 8, the tail 9 inches long.
Petauroides seems to be chiefly distinguished from Petaurus by the fact that, as in its ally Dactylopsila, the tail is partly naked terminally. In Petaurus and Gymnobelideus the tail is bushy to the very end, including its extreme tip below.
A third genus of Flying Phalangers is the minute Acrobates, which has a distichous tail like that of Distaechurus. It is not more than 6 inches in length including the tail. As to these Flying Phalangers it is exceedingly instructive to observe that the same method of "flight" has been apparently evolved three times; for the three genera are each of them specially related to a separate type of non-flying Phalanger. The same observation can be made about the Flying Squirrels, Anomalurus and Sciuropterus. The dental formula is I 3/2 C 1/0 Pm 3/3 M 3/3. The ears are thinly clad with hair. There are four teats.
SUB-FAM. 2. PHASCOLARCTINAE.--The Koala, or Native Bear, Phascolarctos cinereus, is the only representative of its sub-family. It is, like the Wombat, aberrant in the lack of an obvious tail. The absence of this appendage is curious in an arboreal creature whose near allies have a long and prehensile one. The structure of the Koala was investigated by the late Mr. W. A. Forbes.[89] There are some unexpected points of likeness to the Wombat: thus they agree in the absence of the tail, in the structure of the stomach, and in the great subdivision of the lobes of the liver. The brain, however, is smooth, and the caecum is exceedingly large and complicated in structure, that of the Wombat being short. That both animals have cheek-pouches is perhaps due to similar habits of temporarily storing masses of food. This animal has only eleven pairs of ribs. The tail has only seven or eight vertebra, and these have no chevron-bones.
A peculiarity of the skull is seen in the great size of the alisphenoid bulla, which is comparable in size and appearance with that of the Pig. As in the Kangaroos, the atlas is incomplete below. {143}
The tooth formula of the genus is I 3/1 C 1/0 Pm 1/1 M 4/(4 or 5). The additional lower molar seems to be exceptional, and has been found in one specimen only.
In the alimentary tract the most remarkable structure is the large intestine, which is very capacious for the first 28 inches or so of its course. This section of the colon is lined with rugae precisely like those which are found in the caecum. These folds, which at first are some twelve in number, fuse lower down, and by the time that the colon approaches the external orifice are reduced to five. Similar folds, as already stated, occur in the caecum, but do not extend as far as its blind end. The caecum is proportionately and actually larger than in any other Marsupial. The gall-bladder is unusually elongated.
FIG. 70.--Koala. Phascolarctos cinereus. x 1/9.
The Koala is mainly crepuscular or nocturnal in its habits. It feeds so exclusively upon the leaves of the gum-tree (Eucalyptus) that it is impossible to keep the creature long in captivity in lands where that particular kind of food is not available.
The female, though she seems to bear but a single young one, which is carried on the back after the fashion of some Opossums, has two nipples. The animal's slow habits seem to require a nocturnal and retired life. It is about as lethargic as the Sloth, and it is said to further resemble that animal in clinging firmly to a branch even after it is shot. {144}
FIG. 71.--Wombat. Phascolomys wombat. x 1/12.
SUB-FAM. 3. PHASCOLOMYINAE.--Phascolomys, the Wombat, is the only genus of this sub-family. This animal has the appearance of a heavily-built Marmot, like which it has a mere stump for a tail, and a pair of strong chisel-shaped and Rodent-like incisors, which, however, differ from those of Rodents in having a complete coating of cement. All the teeth of the animal are rootless, and there are no canines. The incisors have enamel on the front and lateral faces only. The dental formula is I 1/1 C 0/0 Pm 1/1 M 4/4. The affinities with other Diprotodont Marsupials are shown by the commencing syndactyly of the second and third toes. The rhinarium is naked or hairy. There is a rudimentary cheek-pouch, as in Phascolarctos. The Wombat has, like the Koala, and also the Beaver--which does away with some of the value of the comparison--a peculiar gland-patch in the stomach, a raised area of collected glands. In no other Marsupial is such a structure found, "whilst in the two forms under consideration its identity is almost precise. That such a unique structure should have been independently developed in two forms unrelated to each other, appears to me to be in the highest degree improbable." This is Mr. Forbes' opinion. It might be strengthened by adding the observation that, as there are other points of likeness between the Wombat and the Koala, it seems more unlikely that a structure so nearly identical should have been twice {145} developed in two not very distant forms. As in the Kangaroos, the atlas is open below. Ph. ursinus has 15 ribs; the other species the normal (for Marsupials) 13. Other points of likeness will be mentioned under the description of the Koala. These animals mainly feed upon roots; they live in companies in burrows. There are three species--Ph. ursinus, Ph. latifrons, and Ph. mitchelli. Ph. ursinus is Tasmanian in range, the other two species South Australian.
FIG. 72.--Skull of Wombat. Phascolomys wombat. (Lateral view.) ang, Angular process; cond, condyle of mandible; ex.oc, exoccipital; ext.aud, opening of bony auditory meatus; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; p.max, premaxilla; sq, squamosal; ty, tympanic. (From Parker and Haswell's Zoology.)
SUB-FAM. 4. TARSIPEDINAE.--The genus Tarsipes ought perhaps to be removed from the present family. There is but a single species, which is a small creature of 7 inches in total length, of which the tail measures 4 inches. The teeth are much dwindled, the formula being I 2/1 C 1/0 Pm 1/0 M 3/3 = 22. The lower incisors are procumbent. The lower jaw, moreover, has not the characteristic Marsupial inflection. The intestinal canal is without the caecum present in the remaining Phalangeridae. It is a curious fact that this aberrant little Phalanger should come from Western Australia, like the even more aberrant Myrmecobius. Like the latter also, Tarsipes has a long exsertile tongue, with which, however, it extracts honey from flowers. Probably it also catches minute insects in the corollas of the flowers. It has been proved, in fact, that in captivity at any rate the animal is insectivorous; for it has been known to eat moths.
FAM. 3. EPANORTHIDAE.--The extinct Epanorthidae of {146} Patagonia are represented to-day by a small Marsupial which has been rediscovered within the last two or three years. This little animal, formerly called Hyracodon (a pre-occupied name), is now termed Caenolestes, and is a native of Colombia and Ecuador. There are two species, and of these C. obscurus is called by the inhabitants "Raton runcho," which means opossum-rat. It lives apparently upon bird's eggs and small birds, though it belongs to the Diprotodont division of the Marsupials. Caenolestes, however, although diprotodont, has not the syndactylous character of the digits of the feet already referred to in the Kangaroos and their allies. The pouch is small and rudimentary. The dentition is I 4/3 C 1/1 Pm 3/3 M 4/4 = 46, and the teeth are said by Mr. Thomas to be much like those of the Australian Dromicia.[90]
In the skull a peculiarity which does not bear upon its affinities to other Marsupials, but is still interesting, is mentioned by Mr. Thomas. The nasals are not sufficiently prolonged to meet the upper edge of the maxillae, and so a vacuity is left, as in the skulls of many Ruminants (e.g. the Sable Antelope). The palate is very imperfect; the foramina, which render it so, reach as far forward as the last premolar. The lower jaw has quite the appearance of that of a Macropus or Phalanger, with long and forwardly projecting incisors.
EXTINCT DIPROTODONTS.--The great Diprotodon is a creature with a skull a yard long, which must have been of the size of a large Rhinoceros. Though closely allied to Macropus, it seems that this great beast did not hop after the fashion of a Kangaroo, its limbs being of a more equal size than in the Kangaroo. Recently some further remains of Diprotodon have been discovered in a lake known as Lake Mulligan, where they had apparently been bogged. Professor Stirling has contributed an account of these remains, which fills up a considerable gap in our knowledge. He has been able to state the structure of the fore- and hind-limbs. Both limbs are pentadactyle, the fingers of the fore-limb being approximately equal in length and general development. In the hind-limb the hallux is small, and consists of the metatarsal only. This bone is fixed in the position of "extreme abduction," and is suggestive of an arboreal limb. Digits two and three may have {147} been syndactylous, and the authors of the account[91] of these bones think that the fourth toe may have shared in this syndactyly. The metatarsal of the fifth digit is enormously expanded at its edge, and seems to have furnished a strong support to the creature; this is also seen in the metacarpal of the fore-limb. Probably, therefore, Diprotodon was quadrupedal in its mode of progression, with the emphasis laid upon the little finger and the little toe instead of, as in ourselves, the first toe. The hind-foot of the Diprotodon could not be more unlike that of a Kangaroo than it actually is.
FIG. 73.--Diprotodon australis. (After Owen.)
FIG. 74.--Thylacoleo carnifex. Side view of skull. (After Flower.)
Another giant among these Marsupials was the genus Thylacoleo, whose name was given to it by Sir Richard Owen on the view that it was a Marsupial Tiger. Sir W. Flower has, however, controverted this opinion, and the genus is in fact, in spite of its large size, closely allied to the Phalangers and {148} Cuscuses.[92] The dental formula is I 3/1 C 1/0 Pm 3/1 M 1/2; the last premolar is a great blade-shaped tooth like that of Potorous.
Nototherium was a creature smaller than Diprotodon, but still of large size; it is believed to have been a burrowing creature, and to connect the Wombats with Diprotodon. More certainly allied to the existing Wombat was Phascolonus, a Wombat as big as a Tapir.
FIG. 75.--Nototherium mitchelli. Side view of skull. x 1/6. (After Owen.)
Of extinct American Diprotodonts the Epanorthidae, already referred to in connexion with the living Caenolestes, were the most prominent forms. The genus Epanorthus occurs in the Santa Cruz formation of Patagonia, which is believed to be Miocene. The incisors are three in the upper jaw; and the single incisor of each ramus of the lower jaw is a great chisel-shaped, cutting instrument.
Abderites is also typically Diprotodont by reason of the large projecting incisors of the lower jaw. It has a large cutting tooth in the lower jaw, which appears to be the last premolar, and is thus comparable to the great cutting tooth of the lower jaw and of the upper jaw of the extinct Phalanger, Thylacoleo. {149} It may also be comparable to the great premolar of such Multituberculata as Ptilodus and Plagiaulax. It is, moreover, marked with vertical grooves.
An interesting form, which is unfortunately but little known, is the Australian and Pleistocene genus Triclis, with one species, T. oscillans. In having a minute canine tooth in the lower jaw it agrees with some Phalangeridae, and being otherwise closely allied to Hypsiprymnodon, it unites the Macropodidae with the Phalangeridae.
SUB-ORDER 2. POLYPROTODONTIA.
In this mainly carnivorous or insectivorous division of the Marsupials the incisors are four or five on each side of the upper jaw, and one or two fewer in the lower jaw. Figs. 76 and 77 illustrate the Polyprotodont and Diprotodont dentitions. The canines are those of flesh-eaters and so are the molars, being as a rule sharply cuspidate. As a rule, which has an exception in the Peramelidae, there is no syndactylism of toes in the hind-foot. This sub-order is at the present day Australian and American in its range.
FIG. 76.--Front view of the skull of Tasmanian Devil (Sarcophilus ursinus), showing Polyprotodont and carnivorous dentition. (After Flower.)
FAM. 1. DASYURIDAE.--This family consists of Marsupials which are generally pentadactylous, but with occasionally the hallux missing. The tail is long but not prehensile. The pouch is present or absent. The teeth vary in the different genera, but {150} the upper incisors are never less than three, and may be as many as five in the upper jaw and six in the lower. The canines are trenchant. There is no caecum.
FIG. 77.--Front view of skull of Koala (Phascolarctos cinereus), illustrating Diprotodont and herbivorous dentition. (From Flower.)
FIG. 78.--Longitudinal section of the skull of the Thylacine (Thylacinus cynocephalus). x 1/2. a, Angular process of mandible; AS, alisphenoid; BO, basioccipital; BS, basisphenoid; cd, condyle of mandible; ET, ethmoturbinal; Ex.O, exoccipital; Fr, frontal; ME, ossified portion of mesethmoid; MT, maxilloturbinal; Mx, maxilla; Na, nasal; OS, orbitosphenoid; Pa, parietal; Per, periotic; Pl, palatine; PMx, premaxilla; PS, presphenoid; Pt, pterygoid; SO, supraoccipital; Sq, squamosal; Vo, vomer. (From Flower's Osteology.)
The genus Thylacinus contains but a single species, which is now limited to Tasmania, and is generally known as the Tasmanian Wolf. It has the build of an ordinary Wolf, and is of about the same size. The hinder part of the body is marked with a series of black transverse bands. The hallux is entirely wanting; the pouch opens backwards. The marsupial bones are minute and unossified. The dental formula is I 4/3 C 1/1 Pm 3/3 M 4/4 = 46. There are four mammae. This animal, now confined to Tasmania, {151} is getting rarer on account of its sheep-killing propensities, and the consequent war of extermination declared upon it by the colonists. It will, however, feed upon other animals; and it is related that the first specimen ever captured had in its stomach the remains of an Echidna! Mr. Thomas thinks that the persistence of this and of some of the other larger carnivorous Marsupials in Tasmania after their extinction in Australia is not unconnected with the advent of the Dingo. But it is stated that the Thylacine is quite capable of keeping even a pack of dogs at bay.
FIG. 79.--Tasmanian Devil. Sarcophilus ursinus. x 1/10.
The genus Sarcophilus has been frequently confounded with the next, but it is kept apart by Mr. Thomas, who follows Cuvier in this. An alternative generic name is Diabolus, which, like the first name, refers to the habits and character of the single species which this genus contains. The genus is more like Thylacinus than is Dasyurus. The hallux is wanting, and the teeth, though fewer in number (42), resemble those of the Thylacine more closely than do those of the Dasyure. The species is called S. ursinus, the popular name being Tasmanian Devil. It is black with a variable number of white patches on the body. It is of about the size of a Badger, and is, like the Thylacine, a nocturnal animal. The Tasmanian Devil is said to be one of the most ferocious of animals, and to express its ferocity by a "yelling growl."
{152} [Illustration]
FIG. 80.--Skull of Dasyurus. (Lateral view.) al.sph, Alisphenoid; ang, angular process of mandible; fr, frontal; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; oc.cond, occipital condyle; par, parietal; par.oc, paroccipital process; p.max, premaxilla; s.oc, supraoccipital; sq, squamosal; sq', zygomatic process of squamosal. (From Parker and Haswell's Zoology.)
FIG. 81.--Dasyure. Dasyurus viverrinus. x 1/5. (After Vogt and Specht.)
The next genus of this family, Dasyurus, comprises five species, which range over the whole of the Papuan and Australian sub-regions. The general form is Viverrine, and the hallux is sometimes present though small. The dental formula is as in the last genus, but the teeth "are more insectivorous in their character." There are six or eight mammae. The members of this genus are grey or brown, and spotted with white; they are all arboreal, and feed largely upon birds and their eggs. Mr. Thomas has pointed out that in two species, D. viverrinus and D. geoffroyi, the striae upon the foot-pads are absent, and that therefore these at least are probably not so purely arboreal as the rest. The animals are not diurnal, and during the day hide themselves in the hollow trunks of trees. They are spoken of as "Native Cats," but have the general habits of Martens. D. maculatus is common in Tasmania, but is rare in Australia, thus "approaching the condition now exhibited by the Thylacine and {153} Tasmanian Devil, namely, complete extermination in Australia, where both once lived." D. hallucatus shows an approach to Phascologale in its five-toed hind-feet and slender build.
Phascologale is a genus which, like the last, is usually arboreal (although not P. virginiae of North Queensland), but is of much smaller size, the species not exceeding the dimensions of a rat. They have no spots, but there is sometimes a stripe down the back. There are thirteen species, which have the same range as the last genus. The hallux is present though small, but the pouch is "practically obsolete," though there is a small fold of skin behind the teats. The rhinarium is naked; the tail is long, "bushy, crested, or nearly naked." The mammae are four to ten in number. The dental formula is as in Dasyurus, and the teeth are not very different in form; sometimes the last premolar is wanting. "The members of this genus," remarks Mr. Thomas, "evidently take the place in the Australian region filled in the Oriental by the Tupaiae, and in the Neotropical by the smaller Opossums."
The genus Sminthopsis comprises not more than four species, even smaller than the last. The largest species, S. virginiae, is only 125 mm. in length. The hallux is present, and there is a well-developed pouch. There are forty-six teeth, as in the Dasyures. The feet are narrow with granulated or hairy soles, whereas in Phascologale they are broad with smooth soles. The mammae are eight or ten. The genus ranges through Australia and Tasmania.
The genus Antechinomys has but a single species, which is a native of Queensland and New South Wales. The build is Jerboa-like, and the animal is, as might be inferred, terrestrial. The ears are very long, and the limbs elongated; the hallux is absent; the teeth are exactly as in Sminthopsis.
Antechinomys has thirteen dorsal and seven lumbar vertebrae; three sacrals and twenty-five caudals, the latter number being in excess of that of its allies. The stomach is nearly globular, with approximated orifices; the intestine measured 6.8 inches, a little more than twice the length of the animal itself. A. lanigera is a native of East Central Australia, and appears to be entirely terrestrial in habit, and to progress by a series of leaps--at any rate when going at full speed.
Professor Spencer, who found examples of this rare species, gives {154} an interesting description of its habits. Antechinomys has much the look of the Australian Rat, Hapalotis mitchelli; and as the two animals lead a similar kind of life, the resemblance is not unexpected. Professor Spencer wonders why these creatures are saltatory in habit. The country which they inhabit is arid, but with patches of grass and shrubs. For a big kangaroo the advantage of the power of leaping over such obstacles may be obvious, but not for the small and slender Antechinomys. The chief foes of this rare Marsupial appear to be predatory birds; and Professor Spencer thinks that the saltatory mode of progression may be more baffling to such pursuers than even a rapid run.
The genus Dasyuroides has been lately instituted by Professor Spencer for a Marsupial from Central Australia somewhat intermediate between Sminthopsis and Phascologale. As there is but one species, the generic will be considered with the specific characters. D. byrnei is an animal of about the size of the Common Rat. The hallux is absent. The tail is fairly thick, but not "incrassated." There are six mammae, and the pouch is but slightly developed, with two low lateral folds. The dentition is I 4/3 C 1/1 Pm 3/2 M 4/4. This Marsupial is nocturnal, and burrowing in habit. Its food consists of insects.[93]
Myrmecobius is so different from the last-described genera (DASYURINAE) that it is usually separated from them as a sub-family MYRMECOBIINAE. The animal is of a bright rufous colour, banded posteriorly with white. There is no hallux, though the metatarsal belonging to that digit is present. There are four mammae.[94] On the chest is a naked patch of some extent, upon which open the ducts of a complex gland, which has been described and figured by myself.[95] There is no pouch, but a tract of skin shows indications of a pouch-like structure. The teeth are extraordinarily numerous, fifty to fifty-four; the formula being I 4/3(4) C 1/1 Pm 3/3 M 5/6. Their resemblance to those of certain Jurassic Marsupials is dealt with on p. 100.[96] In this matter lies of {155} course the chief interest of the genus, which may be "an unmodified survivor from Mesozoic times, and therefore from a time long before the Didelphyidae, Peramelidae, and Dasyuridae were differentiated one from the other." Another ancient feature (found in Jurassic mammals) is a mylo-hyoid groove upon the lower jaw, which, however, is not always present, and its existence has therefore been denied. The single species, M. fasciatus, is partly arboreal and partly terrestrial in habit, and feeds upon ants. It is a Western and Southern Australian form.
FIG. 82.--Banded Australian Anteater. Myrmecobius fasciatus. x 1/5.
FAM. 2. DIDELPHYIDAE.--All the members of this family are pentadactylous. The teeth are fifty in number, arranged thus: I 5/4 C 1/1 Pm 3/3 M 4/4. The caecum is small; the pouch is generally absent; the tail generally long and prehensile.
FIG. 83.--Virginian Opossum. Didelphys virginiana. x 1/5. (After Vogt and Specht.)
The genus Didelphys contains most of the forms belonging to this family, including as it does some twenty-three species. The Opossums are mainly arboreal animals, insectivorous in their food; but the larger species eat reptiles, birds, and their eggs. Several of the small species carry their young, when able to leave the teats, on {156} their back, the tails of the young being wrapped round that of the mother. It is not only the pouched species which carry their young in something of this fashion. Azara's Opossum, an animal as big as a cat, is said to carry its eleven young ones (themselves as large as rats) on the back, though their foothold does not appear to be strengthened by intertwining the tails. Even with this huge family on her back, the mother can climb trees with considerable alacrity. The mammae are seven to twenty-five in number. The genus has been lately split up into a number of genera, Marmosa, Dromiciops, Peramys, etc.
FIG. 84.--Thick tailed Opossum. Didelphys crassicaudata. x 1/5.
Chironectes is hardly different from Didelphys. It has webbed hind-feet, and is aquatic in habit. The one species of the genus is known as the Yapock, and is a Central and South American form. It is of about the size of a large rat, and appears to be an expert diver after the fish upon which it lives.
FAM. 3. PERAMELIDAE.--The Bandicoots, although clearly belonging to the Polyprotodont Marsupials, yet agree with the Diprotodonts in the fact that the second and third toes of the feet are bound up in a common integument, which is not the case with the Diprotodont Caenolestes. The hind-feet are longer than the front; of the former limb, two or three of the fingers alone are long and functional; the others are rudimentary or absent. Tail long, hairy, and non-prehensile. Dentition I 5/3 C 1/1 Pm 3/3 M 4/4 = 48, or sometimes, owing to the absence of a pair of upper incisors, 46. There is a caecum.
{157} [Illustration]
FIG. 85.--Bones of manus. A, of Choeropus castanotis. x 2. B, of Bandicoot (Perameles). x 1-1/2. c, Cuneiform; l, lunar; m, magnum; R, radius; s, scaphoid; td, trapezoid; tm, trapezium; u, unciform; U, ulna; I-V, digits. (From Flower's Osteology.)
FIG. 86.--Rabbit Bandicoot. Peragale lagotis. x 1/5.
The genus Peragale, the Rabbit-Bandicoots, consists of two species entirely Australian in range. The enormous ears (whence "Rabbit" Bandicoot) distinguish this genus from Perameles. The pouch opens backwards, and there are eight mammae. P. lagotis, the only species about whose ways of life anything is {158} known, burrows in the soil, whence it extracts grubs; it is also a grass-feeder, and it is said that its likeness to a Rabbit in appearance is strengthened by its similarity in flavour!
Perameles is a genus consisting of twelve species, which are found in Tasmania, Australia, and New Guinea. Like the last genus, from which it does not widely differ in other points, Perameles consists of species which combine insectivorous and vegetarian habits. One species is said to become in captivity an expert in catching mice. The pouch opens backwards, and there are six or eight mammae.
FIG. 87.--Pig-footed Bandicoot. Choeropus castanotis. x 1/3.
The last genus of this family is Choeropus, containing but one species, Ch. castanotis. It is confined to the Australian continent. It is to be distinguished from the last two by the fact that there are only two functional digits, the second and third, in the fore-limb; the fourth is rudimentary; the other two are absent. It burrows, and is omnivorous like its allies. The two metacarpals that are developed are very long and closely apposed; they have hence a remarkably pig-like aspect, and justify its name. The pouch opens backwards, and there are eight mammae.
FAM. 4. NOTORYCTIDAE.--This family contains but a single genus and species, the recently-discovered Notoryctes typhlops.[97]
{159} We may regard as family-characters the pentadactyle limbs, the existence of three pairs of incisors in the lower and four in the upper jaw; and the tritubercular nature of the upper molars. Notoryctes typhlops, the "Marsupial Mole" as it has been termed, was originally discovered by Professor Stirling in Central South Australia. It is a burrowing creature, clothed in a silky fur of a pale golden red, without external ears. It has been compared in appearance with Chrysochloris, the Cape Golden Mole, and the eminent palaeontologist, Professor Cope, has even insisted upon a real genetic affinity. Edentate affinities have also been suggested. But Notoryctes has a small pouch opening backwards as in other Polyprotodonts,[98] and as it also possesses marsupial bones it must undoubtedly be referred to the Marsupialia. The animal shows many curious adaptations to its underground mode of life. Certain of the vertebrae in the neck and in the lumbar region are firmly welded together, giving of course a strength of push, and suggesting the Armadillos; the claws of the third and fourth front-toes are greatly enlarged, and must be efficient digging organs. The track of the animal is like that of a railway in mountainous country; it burrows for a short distance, emerges, and then descending beneath the surface re-emerges. The red colour of the fur is said to be in harmony with the arid soil in which it lives. The native name of the creature is "Urquamata." It feeds upon ants and other insects.
FIG. 88.--Australian Marsupial Mole. Notoryctes typhlops. x 1/4.
EXTINCT POLYPROTODONTS.--Of extinct Polyprotodonts (apart from those Mesozoic forms which are considered on p. 100) extinct species of Thylacinus and Dasyurus are known from {160} Australia. The most interesting fact in connexion with the Tertiary Polyprotodonts is the existence in South America of such genera as Prothylacinus and Amphiproviverra, which are not merely Polyprotodonts but definitely Dasyures, and not referable to the Didelphyidae.
These forms have been included in an order, SPARASSODONTA. But it is not by any means certain whether these forms are rightly placed in the neighbourhood of the carnivorous Marsupials; it is possible that they ought to be relegated to the Creodonta or to their allies. Their structure is in fact somewhat intermediate between those two groups. The teeth seem to be carnivorous and Marsupial-like in form; but as already mentioned, in connexion with the general structure of teeth, more than a single premolar is replaced. These animals in fact, in so far as regards their teeth, are midway between the Marsupials and the typical Eutheria. The angle of the lower jaw is inflected, but the palate is not marked by deficient ossification. At least this is not the case with all the members of the group. Whether the small Microbiotherium, which is made the type of a family, is rightly referred here is not certain. This animal had palatine vacuities as well as an inflected angle to the lower jaw.
* * * * *
{161}
VIII. EDENTATA--GANODONTA
ORDER II. EDENTATA
Terrestrial, partly subterranean, or arboreal creatures of quite small to gigantic size (some extinct genera), with frequently a covering of scales or bony scutes. Limbs clawed. Teeth either totally absent or, if present, imperfect in structure, being without enamel, and not forming a complete series; incisors and canines being as a rule absent. Teats axillary, pectoral, or inguinal.[99] Retia mirabilia very common in the extremities.
To this group the name of Bruta was given by Linnaeus, but then it included not only the families which we now place in the modern order Edentata, but also the Elephant and the genus Trichechus. Mr. Thomas has proposed to change the name into Paratheria, which name is suggestive of what he and some others think concerning the systematic position of the group, i.e. that it is not to be placed in the Eutherian group of mammals at all, but represents a separate twig which has arisen with the Eutheria from a low mammalian stock. This view can hardly be accepted if the Ganodonta--which will be treated of presently--be really ancestral Edentates, for they are not in any way a Prototherian mammalian group, so far as their remains enable us to judge.
The Edentata contain the Sloths, Ant-bears, Armadillos, Manis and Orycteropus, among living forms. The great Ground-Sloths, Megatherium, etc., and Armadillos, Glyptodon, etc., represent the extinct forms.
The name that has been applied to this group is inappropriate {162} inasmuch as many Edentates have teeth. It is, however, by a number of small tooth-characters that the order can be defined. Thus if teeth are present they are simple in structure, without enamel in the adult condition, though a rudimentary enamel-organ has been discovered in an Armadillo. The teeth, moreover, are not found in the anterior part of the mouth, and they grow from persistent pulps; neither is there much differentiation among them. It is not possible, however, to speak of the Edentates as quite homodont, since in Orycteropus there are large cheek-teeth; but there is at any rate not a marked heterodonty in that or in any other Edentate. It used to be said that the Edentates were monophyodont. But the Armadillo Tatusia was subsequently found to possess a second suppressed dentition, and after this discovery Mr. Thomas proved that Orycteropus is also diphyodont. Since then other Armadillos have been shown to be diphyodont; and the whole group therefore, so far as concerns those members that have teeth, may in all probability be regarded as typically mammalian in this respect.
These characters are slender enough, but there seem to be no others by means of which the members of this order can be satisfactorily linked together. The fact is, that we have here a polymorphic order which contains in all probability representatives of at least two separate orders. We have at present a very few, and these perhaps highly modified, descendants of a large and diverse group of mammals. For convenience' sake they will be all treated of under the head of Edentata.
Although for the probable reasons already stated it is a hard matter to frame such a definition as will include all existing Edentates, it is easy enough to define two groups in this heterogeneous order; to define one group we should say, rather, and then to regard the leavings as forming another not so easily definable a group.
The perfectly-definable group is that which includes the American Anteaters, the Armadillos, and the Sloths. In all these creatures, which may certainly be regarded as representing on their own account as many family types, there are a number of important and highly-characteristic anatomical features which they share in common. So exceedingly different are these three types in general appearance and (correlated with that) in way of life that these common characters acquire increased importance. {163}
FIG. 89.--Great Anteater (Myrmecophaga jubata). A, Side view of twelfth and thirteenth thoracic vertebrae. B, Posterior surface of second lumbar vertebra. C, Anterior surface of third lumbar vertebra, x 2/3. az, Anterior zygapophysis; az^1, az^2, az^3, additional anterior, articular facets; cc, facet for capitulum of rib; m, metapophysis; pz, posterior zygapophysis; pz^1, pz^2, pz^3, additional posterior articular facets; t, transverse process; tc, facet for articulation of tubercle of rib. (From Flower's Osteology.)
The first of these characters is the series of additional zygapophyses on the posterior dorsal and lumbar vertebrae; these are very clear in the Anteaters and Armadillos; less clear, but still obviously represented, in the Sloths. In the second place, they all possess a clavicle, rudimentary, it is true, in the Great Ant-bear, but still present. Thirdly, the testes are abdominal throughout life, a character which they share with such lowly-organised animals as the Monotremata and the Whales. Finally, and this is by no means a matter to be overlooked, not only are all the existing members of this group American in range, but there is no evidence to prove that they have ever existed elsewhere. No European or Old-World {164} representatives have as yet been discovered which can be referred to the Anteater, Armadillo, or Sloth type with certainty.[100]
Of these American forms, which will be treated of first, the Armadillos are further apart from either Sloths or Anteaters than the last two are from each other. The name XENARTHRA has been suggested for the American Edentates with "abnormal" vertebral articulations; the corresponding NOMARTHRA includes the Old-World forms.
FIG. 90.--Right scapula and clavicle of Two-toed Sloth (Choloepus hoffmanni). x 1-2/3. a, Acromion; af, prescapular fossa; c, coracoid; cl, clavicle; csf, coraco-scapular foramen; gc, glenoid cavity; pf, postscapular fossa. (From Flower's Osteology.)
Between the Sloths and Anteaters the extinct Megatherium and some of its allies are to a certain extent intermediate. But it may be pointed out in the first place that there are certain important resemblances between the living forms. In both, retia mirabilia are developed in the tail (in spite of its reduction in the Sloths) and in the limbs. But, as is well known, retia are also found in other mammals far removed in the series from these under consideration. The reproductive organs generally are very similar, and they have both a dome-shaped and deciduate placenta. The latter character they share with the Armadillos and with the Aard Vark; Manis having a non-deciduate placenta which is, like that of the Carnivora, zonary in form. The Edentates, at any rate the American forms, have a double vena cava posterior and no azygos vein. This condition is also met with among Whales.
Osteologically the Sloths and Anteaters are united by the fact that the coracoid becomes fused with the coracoid border of the scapula, thus forming a foramen; the importance of this character is, however, discounted by its occurrence in three genera of Cebidae.
The above facts embody the views of Sir William Flower.[101]
{165} A subsequent study of the brain and of the muscles of these animals has led to results not entirely in harmony with these views.
Dr. Elliot Smith is of opinion,[102] after an exhaustive study of the Edentate brain, that in this region of the body the present group shows very decided points of likeness to the Carnivora; that is, so far as concerns the Anteaters. On the other hand, Orycteropus is as distinctly comparable with a primitive Ungulate type, such as is exemplified by Moschus. "If the brain of Orycteropus," he remarks, "were given to an anatomist acquainted with all the other variations of the mammalian type of brain, there is probably only one feature which would lead him to hesitate in describing it as an exceedingly simple Ungulate brain. That one feature is the high degree of macrosmatism.[103] Manis, on the other hand, does not come especially near to Orycteropus. The brain of Manis conforms to a simple type of architecture, which agrees in many points with both those of Orycteropus and the American Edentates; there is not sufficient evidence to show which type it really favours." Elliot Smith would, in fact, agree with Max Weber that it is better, if a division is to be made, to divide the group into three orders:--the Xenarthra (Sloths, Anteaters, and Armadillos), Tubulidentata (Orycteropus), and Squamata (Manis), instead of into Xenarthra and Nomarthra.
Messrs. Windle and Parsons[104] are disposed to see in muscular similarities reasons for uniting Manis with the American Edentates, though they confess to being unable to place Orycteropus; in this animal, they say, "we are more struck by the generalised mammalian arrangement of its muscles than by any special Edentate characters. There are, however, two muscles in Orycteropus which show peculiarities not found elsewhere than in the Edentates";--the triceps, which has more than one scapular head, and the tibialis posticus, which is double. They conclude that Orycteropus "presents some feeble claims to be taken into the order."
We shall here adopt the following divisions. {166}
SUB-ORDER 1. XENARTHRA.
FAM. 1. MYRMECOPHAGIDAE.--The family Myrmecophagidae contains three genera, all South American in range. These genera, Myrmecophaga, Tamandua, and Cycloturus, agree greatly in their outward form. They are all without teeth, and have long snouts and long protrusible tongues. The fur is thick, and they have powerful claws wherewith to break down the strong ant-hills upon whose inhabitants they feed. Tamandua and Cycloturus are arboreal, Myrmecophaga is terrestrial in habit. The claws of the arboreal forms are useful to destroy the bark, and thus bring to light insects which lurk in such situations.
FIG. 91.--Great Anteater. Myrmecophaga jubata. x 1/10.
The genus Myrmecophaga contains but one species, the Great Anteater, Myrmecophaga jubata. It is a large and handsome animal, with long, shaggy, greyish-black hair and a broad white stripe across the shoulder. The coloration is similar in the two sexes. Including the long and bushy tail it reaches a length of over 7 feet. It is on account of its long tongue and greatly developed salivary glands that this and the allied genera were originally placed with Manis. It is the submaxillary glands which are so enormous; they extend back over the chest, and open by three distinct ducts, of which two unite just before the external orifice. {167} Along their course these ducts are provided with a sphincter muscle, which squeezes the secretion towards the external orifice into the mouth-cavity. The stomach is somewhat gizzard-like. The intestine has no caecum.[105]
The Anteater's great claws are not only serviceable in tearing up the ground to get at its food; armed with them he does not fear, as Mr. Waterton remarked, "the fatal pressure of the serpent's fold or the teeth of the famished jaguar." An Anteater, too, is more than a match for a big dog, and will rip open its belly with the claws while the dog is vainly trying to make an impression with its teeth upon the shaggy hair.
Tamandua is a smaller animal than Myrmecophaga, and, as has been stated, is arboreal; associated with this habit is a prehensile tail. Like the last genus, Tamandua has a rudimentary clavicle, this bone being well developed in the little Cycloturus.
FIG. 92.--Skull of Anteater (Myrmecophaga). Lateral view, al.sph, Alisphenoid; cond, condyle of mandible; cor, coronoid process of mandible; ex.oc, exoccipital; ext.aud, external auditory meatus; fr, frontal; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; occ.cond, occipital condyle; pal, palatine; par, parietal; p.max, premaxilla; s.oc, supraoccipital; sq, squamosal; ty, tympanic. (From Parker and Haswell's Zoology).
The skull of the Anteater[106] is very long and low; the fore-part is tubular, and there appear to be no traces of teeth. The premaxilla is very small; the zygomatic arch is imperfect, and does not reach the squamosal behind. A curious feature of this genus, which it shares with some Dolphins and other Whales, is that the pterygoid bones develop palatine plates which meet each other in the middle line, and thus shift the opening of the {168} posterior nares backwards. This is also, of course, a character of various lower vertebrates. Another Whale-like character in the skull is the weak character of the mandible, which does not give off a marked coronoid process. But then in neither group is there much mastication. The tympanic, periotic and squamosal are ankylosed together. A peculiarity of the cervical vertebrae is that (as in the Camels) the vertebrarterial canal of several of the vertebrae perforates the pedicle obliquely. There are fifteen or sixteen dorsal and three or two lumbar vertebrae. The additional zygapophyses upon the former have been already referred to. The mode of articulation of the ribs is highly singular.
FIG. 93.--Skull of Anteater (Myrmecophaga). Ventral view. Letters as in Fig. 92. In addition, b.oc, basioccipital; glen, glenoid surface for mandible; pter, pterygoid. (From Parker and Haswell's Zoology.)
FIG. 94.--Side view of three mesosternal segments of a young Anteater (Tamandua), showing the mode of articulation of the sternal rib (sr). mst, The upper or inner surface of the mesosternal segment; sy, the synovial articulation between the segments. (From Flower's Osteology, after Parker.)
Each segment of the sternum (of which there are eight) is separated from the next by a synovial membrane: and it has on either side two facets for articulation with the ribs. The way in {169} which these latter bones are connected with the sternum is curiously like their mode of connexion with the spinal column at their other end. With this may be possibly compared the double articulation of the single rib (which articulates with the sternum) in the Rorquals. In Cycloturus this mode of articulation does not occur.
The manus of Myrmecophaga is five-fingered. Of these the third digit (as in Perissodactyles) is the most prominent; it is at least double the width of the second or third finger; the pollex is very slender. In the little Cycloturus this is carried to a greater extent: the third digit is relatively enormous; the first and the fourth have become quite rudimentary; while the fifth is only just recognisable as a minute ossification.
FIG. 95.--A, Manus of Great Anteater (Myrmecophaga jubata). x 1/3. B, Manus of Little Anteater (Cycloturus didactylus). x 2. c, Cuneiform; l, lunar; m, magnum; p, pisiform; s, scaphoid; td, trapezoid; tm, trapezium; u, unciform; I-V, digits. (From Flower's Osteology.)
The chevron-bones in the tail surround a well-developed rete mirabile, a rete being found in precisely the same position in the Eastern Manis. Tamandua has also retia, which are also found in the Spider-monkeys.
Cycloturus is by far the smallest of the Anteaters. It has {170} only two toes on the fore-feet. It is to be distinguished, anatomically, from its larger relatives by the complete clavicle, and by the fact that the pterygoids do not meet in the middle line of the skull. The ribs, too, are unusually wide, as in the Whale Neobalaena, and form a bony encasement for the body. It has two small caeca. Of fossil Anteaters but little is known. The most interesting form is Scotaeops, interesting because it has two small back teeth, which are totally lost in its living allies. The huge Patagonian extinct bird Phororhacos, first known by a lower jaw, was at one time regarded as a member of this group on account of the form and edentulous character of the jaw.
FIG. 96.--Unau, or Two-toed Sloth. Choloepus didactylus. x 1/5. (After Vogt and Specht.)
{171} [Illustration]
FIG. 97.--Skull of Three-toed Sloth. Bradypus tridactylus. Lateral view. fr, Frontal; ju, jugal; lcr, lachrymal; max, maxilla; nas, nasal; par, parietal; s.oc, supra-occipital; ty, tympanic. (From Parker and Haswell's Zoology.)
FAM. 2. BRADYPODIDAE.--The Sloths, genera Bradypus and Choloepus, come, as already stated, very near to the Anteaters, in spite of their striking difference in appearance. The Sloths are purely arboreal creatures, with strong recurved claws, which serve as hooks to keep them suspended from the lower side of a branch. The three-toed sloth, Bradypus (or "Ai"), has the exceptional number of nine cervical vertebrae; the two-toed sloth, Choloepus hoffmanni (or "Unau"), has the equally exceptional number of six. The hair is long and shaggy, and gets an adventitious green colour from the presence of minute algae.[107] This gives to the animal the appearance of a lichen-covered bough, a resemblance which is increased in one species by an oval mark upon the back, which suggests forcibly a broken end of such a branch. The likeness of a Sloth to its surroundings is pointed out by Dr. Siemann,[108] who observed that a species occurring in Nicaragua "has almost exactly the same greyish-green colour as Tillandsia usneoides, the so-called 'Vegetable Horsehair' common in the district.... If it could be shown that it frequented trees covered with that plant ... there would be a curious case of mimicry between the sloth's hair and the Tillandsia, and a good reason why so few of these Sloths are seen." The stomach in the Sloths is complicated in structure, with several chambers; one of these gives off a long crescent-shaped caecum. The skull of the Sloths agrees in a number of particulars with that of the Anteaters. {172}
FIG. 98.--Skeleton of Three-toed Sloth. Bradypus tridactylus. (After de Blainville.)
The zygoma is incomplete, though the part connected with the frontal has a strong downward process like that found in Diprotodon and some other mammals. There is, moreover, a process from the squamosal, though it does not reach the anterior part and thus {173} complete the arcade. The premaxillaries are very small, and are usually lost in dried skulls. Coupled with these points of likeness are some differences. The lower jaw, for instance, has a well-marked coronoid process. The pterygoids do not meet in the middle line. The teeth are five or four in each half of each jaw. There is no trace of a second set.
A peculiarity of the Sloths is the enormous number of dorsal vertebrae. There are twenty-three of these in Choloepus hoffmanni, but only fifteen to seventeen in the Three-toed Sloth, Bradypus. As in other American Edentates, the acromion joins the coracoid. This connexion occurs in both the Two-toed and the Three-toed species. The limbs of these creatures are very long, a concomitant of an arboreal life. The femur has no third trochanter. The genus Bradypus, which by reason of the fact that it has not lost the third toe on the manus seems to be more primitive than Choloepus, shows another structural feature which does not bear out this conclusion. The trapezoid and the os magnum of the carpus are united, while in Choloepus they are perfectly distinct bones.
The intestine has no caecum.
There are several species of Sloths. Eminently perfect though the organisation of the Sloth in relation to its particular surroundings appears to us, Buffon selected the animal as the very type of imperfection in nature. "One more defect," he wrote, "they could not have existed."
FAM. 3. DASYPODIDAE.--The family Dasypodidae or Armadillos contains a considerable number of genera. Tatusia, Tolypeutes, Dasypus, Xenurus, Priodon,[109] and Chlamydophorus. They have all a more or less rigid covering of bony plates imbedded in the skin, which are not in the least comparable with the scales of the Manis. Save the Whales, in one or two genera of which traces of a dermal armature exist, the Armadillos are unique among existing mammals in this particular. The term "Edentate" is especially inapplicable to the Armadillos; the genus Priodon may have more than forty teeth in each jaw; a total of ninety was found in one specimen examined by Professor Kuekenthal. In the tendency of the teeth to multiply, we have another example of a state of affairs which characterises so many Whales. Generally, however, seven to nine is the number of teeth in each {174} half jaw, of which one is often implanted in the premaxilla. The Armadillos show their alliance with the other American Edentates in the points enumerated above. Their teeth specially ally them to the Sloths, while the salivary and digestive organs generally are on the Anteater plan, but present a less extreme development. There are, however, caeca, paired as in birds, in the genera Dasypus and Chlamydophorus. The others have none. But there is a dilatation at the commencement of the large intestine, which is not very different from the slightly-developed caeca of Dasypus.
There are certain peculiarities in the skeleton, which distinguish this family.
FIG. 99.--Skull of Armadillo. Dasypus sexcinctus. x 2/3. ex.oc, Exoccipital; fr, frontal; max, maxilla; nas, nasal; par, parietal; peri, periotic; p.max, premaxilla; s.oc, supraoccipital; sq, squamosal; ty, tympanic. (From Parker and Haswell's Zoology.)
{175}
FIG. 100.--Bones of the right manus of the Hairy Armadillo. Dasypus villosus. x 2/3. c, Cuneiform; l, lunar; m, magnum; p, pisiform; R, radius; s, scaphoid; td, trapezoid; tm, trapezium; u, unciform; U, ulna; I-V, digits. (From Flower's Osteology.)
FIG. 101.--Bones of the manus of the Great Armadillo. Priodon giganteus. x 1/3. a, An accessory carpal ossicle in front of the pisiform, which is not seen in the figure. Other letters as in Fig. 100. (From Flower's Osteology.)
The skull in the Armadillos presents a number of likenesses to the other American Edentates.[110] The premaxillaries are small, but are larger in Dasypus than in Tatusia. On the other hand the lachrymals are larger in the latter. The zygomatic arch is complete, but there is no downward process as in the Sloths. In Tatusia (but not in Dasypus) the "short thick pterygoids add somewhat to the hard palate." This is clearly a beginning or a remnant of the quite crocodilian character of the palate of Myrmecophaga. In the cervical vertebrae we see the Whale-like character of fusion between individual vertebrae; and also, as in the Whales, the degree to which this fusion is carried out varies; two to four may be thus united. The additional articular facets upon the dorsal vertebrae have been already commented upon as a point of important likeness to other American Edentates. The dorsal vertebrae are commonly eleven in number, the lumbar being three. But in Priodon the numbers are twelve and two respectively. There are traces to be observed of the double-headed attachment of the ribs to the sternum. The shoulder girdle of the Armadillos is somewhat diverse in form in different genera; the acromion is always large, and is remarkable in Priodon for the fact that the humerus also articulates with it, its extremity being recurved, and forming a socket for this purpose. As in some other Edentates there is a second spine on the scapula behind the first. The clavicle is strong. There is some variation in the form of the manus. It is five-fingered in Dasypus; in Tolypeutes the first digit has vanished; on the other hand, in Priodon, the fifth has become rudimentary {176} and the third enormously enlarged. This latter fact recalls the arrangement characteristic of Myrmecophaga. The pelvis is greatly attached by the ischium to the vertebral column. The femur has a third trochanter.
The various forms of Armadillos are largely distinguished by the number of movable thin bands of scutes lying between the large anterior and posterior shields. Thus we have Dasypus sexcinctus, Tolypeutes tricinctus, etc.
FIG. 102.--Pelvis and sacrum of Armadillo. Dasypus sexcinctus. ac, Acetabulum; il, ilium; isch, ischium; obt.for, obturator-foramen; pect.tub, pectineal tubercle; pub, pubis. (From Parker and Haswell's Zoology.)
The little Pichi-chago (or, more correctly, Pichy-ciego), Chlamydophorus, which only grows to about 5 inches in length, has no movable bands at all. It is covered with a uniform series of plates, which, moreover, are not discontinuous at the neck. It differs, too, from the prevailing Armadillo-type by the absence of conspicuous external ears. In the anterior part of the body the armature consists of little more than the horny plates, which in other Armadillos overlie the bony dermal plates. In the hinder region the bony plates are strong. In this animal, therefore, we have the dermal armature reduced to a minimum; but it must be noticed that, like the extinct Glyptodons, the armature is continuous and nowhere ringed.
The genus Tolypeutes, of which the best-known species is T. tricinctus, the Apar (there are two other species in the genus), can roll itself up into a ball like the Pill-Millipede (Glomeris), and, protected by its armour, roll away from its enemies like the Arthropod under similar circumstances. This mode of protection, be it observed, is also adopted by the Pangolin and by the {177} Hedgehog. The genus has only three movable bands. The tail is short, and is covered with large tubercles. This genus is very markedly digitigrade when running.
FIG. 103.--Three-banded Armadillo or Apar. Tolypeutes tricinctus. x 1/4.
FIG. 104.--Peludo Armadillo. Dasypus sexcinctus. x 1/4. (After Vogt and Specht.)
The Peludo, Dasypus sexcinctus, is, like other Armadillos, an omnivorous creature, and appears to be particularly fond of carrion. It will burrow up to a decaying carcase like the ground-beetles. {178} Mr. W. H. Hudson has described the way in which this Armadillo will kill a snake by holding it down and literally sawing the reptile in half by help of the sharp and serrated edges of the carapace. Dasypus has a very short tail, which is shielded by distinct rings near the base.
Tatusia novemcincta is a species with nine movable bands. The genus has four teats; the ears are near together. There are no caeca and no azygos lobe to the lung. A species apparently belonging to this genus, but described under the generic names of Cryptophractus and Praopus, is remarkable for the thick covering of hair, not entirely wanting but usually thin in other Armadillos. In this particular species the coat of hair is so thick as to conceal the underlying plates of the carapace. The individual hairs are stiff, and one inch and a half in length.[111]
The genus Xenurus contains several species, the best known of which is inaptly named X. unicinctus. As a matter of fact the characteristic feature of the genus is the existence of twelve or thirteen movable plates between the two ends of the body. X. unicinctus has twelve dorsal and three lumbar vertebrae. This Armadillo, known by the vernacular name of the Cabassou, has one of the most modified hands that are found in the family. The first two digits are slender and elongated; but are quite normal in the number of their phalanges. In the remaining three digits the metacarpal is short and broad, while the proximal phalanx is either suppressed altogether or fused with the metacarpal, the middle phalanx is present but short, while the third phalanx is very large indeed. As in Dasypus, but not as in Tatusia, which is in so many other respects divergent from these genera, the lungs have an azygos lobe. As a small point of difference, tending to show an alliance between the genera Xenurus and Dasypus and their difference from Tatusia, is the deeply-imbedded gall-bladder; this sac is not nearly so deeply plunged into the hepatic tissue in Tatusia. Xenurus has no caecal dilatations. The brain "is intermediate in its form and surface markings between Dasypus and Tolypeutes." The small intestine is nearly eighteen times the length of the large. But these intestinal measurements are not of much avail in this group as marks of affinity, since in three species of Dasypus Garrod gives the following widely-divergent lengths:--D. villosus, 11.5 feet and 1.25; D. minutus {179} 5.1, with a large intestine of no less than 7 feet; D. vellerosus 4.3 and .66.
Priodon is the giant of its race. This Armadillo may reach a length of 3 feet to the base of the tail. The tail is some 20 inches long. The large number of teeth has been already noticed. There are twelve or thirteen bands. Other points in the structure of this genus have already been mentioned, and need not be recapitulated. This Armadillo feeds upon termites and carrion.
Scleropleura is unfortunately but imperfectly known. The single species, named by Milne-Edwards[112] S. bruneti, is apparently a very rare inhabitant of Brazil. It is known by a single skin, which was tanned by the hunter who obtained it. Thus the hair, if any, has dropped out. The plates in the skin are deficient along the back and even upon the top of the head, and are barely represented upon the tail posteriorly. The ears are small and distant from each other. The tail is longish, about one-third of the length of the body. The total length of the creature including the tail is rather more than a foot and a half. The hunter who obtained it regarded it as a hybrid between an Armadillo and an Anteater.
EXTINCT XENARTHRA.--There are a good many extinct forms of Armadillo, apart of course from the Glyptodons. Peltephilus is referred to later (p. 186). Dasypus was represented by a large form, 6 feet long, with a skull of one foot in length. The genus Eutatus was also large. The carapace was formed of thirty-three distinct bands, of which the last twelve are soldered together, but not fused into a shield as in Dasypus, etc.
An extinct group of American Edentates, termed the GRAVIGRADA,[113] are somewhat intermediate between the Sloths and the Anteaters. A number of the genera are well known from complete skeletons.
One of the typical forms of this group is Mylodon, which, together with its immediate allies, is often placed in a separate family, MYLODONTIDAE.
Mylodon itself was a large creature, as big as a Rhinoceros. It was covered externally by armour in the skin, which did not form a massive armature as in the Glyptodonts, but was in the {180} form of scattered plates, small and not fused together. The general aspect of the skull is decidedly Sloth-like. As in that animal, the malar bone is bifid posteriorly, and between the bifurcation is embraced the process of the squamosal. This latter is thus more developed than in the Sloth, but there is no actual union between it and the malar. The premaxilla is small. The lower jaw has both coronoid and ascending processes, and is massive. There are five teeth on each side above, and four on each side below, as in the Sloths. There are the normal seven cervical vertebrae and sixteen dorsals. The limbs are not long and slender,but short and strong, the animal having been terrestrial. The fore-feet were five-toed, of which the three inner toes had claws. The hind-feet were only four-toed, and the two inner only were clawed.
FIG. 105.--Mylodon robustus. (Restoration, after Owen.)
Scelidotherium is a genus which is a trifle smaller than the last. It has only four properly-developed toes in the fore-foot, the thumb being rudimentary; of these, the first two bear claws. The hind-feet are also four-toed. Like Mylodon, Scelidotherium is a Pleistocene genus.
Glossotherium has a skull very much like the last two genera; but it is remarkable for the fact that the nostrils instead of being unprotected with bone anteriorly are there closed by a plate of bone formed by the well-developed premaxillae, the nostrils appearing at the sides, and giving the skull a curious likeness to that of a Chelonian. From a series of recent and most important observations it appears to be clear that this genus has survived into quite modern times.[114]
{181}
The well-known naturalist of La Plata, Senor Moreno, engaged in studies connected with the political boundary line between Chili and the Argentine, had occasion to visit Consuelo Cove on Last Hope Inlet in Patagonia. Hanging from a tree he noticed a piece of dried skin, which at once struck him as looking more like the remains of a Mylodon than of any living animal. The inhabitants regarded this piece of skin as a great curiosity, but were of opinion that it was the hide of a cow encrusted with pebbles! This fragment from a bygone age was originally described by Professor Ameghino, who had apparently seen some of the bonelets imbedded in it, as Neomylodon listai, "a living representative of the ancient Gravigrade Edentates of Argentina." That this piece of skin is of quite recent date seems to be proved by a number of considerations. In the first place it is covered by long hair of a light yellowish-brown colour; it does not seem likely that hair would preserve its character for geological epochs. The nearest corresponding case is that of the remains of Moas in New Zealand, whose feathers, dried skin, and tendons are known. Now the Moa was unquestionably contemporaneous with man, as abundant surviving legends prove, and indeed it cannot have been long extinct. Still, hair is a resisting structure, and in a dry cave, with no possibility of irruptions of floods, might retain its characters for long periods. The evidence, however, of more recent date is stronger than this. The skin shows patches of reddish colour, suggestive of course of blood-stains. A small piece of the outside of the skin at the cut edge, which presented the appearance of freshly or comparatively freshly dried fluid, was submitted to a chemical examination and shown to be serum! Dr. Loennberg examined chemically a bit of the skin itself and found in it, after boiling, glue, "which proves that the collagen and gelatinous substances are perfectly preserved." After this it seems impossible to suppose that the skin can be of any very great age; for bacteria would have finished their work upon the serum and gelatine long ago. Combined with the fresh appearance of the skin is the very fresh appearance of the skull. In fact it is impossible to believe that the animal was not alive quite a few years since, relatively speaking. It is admitted that this animal was contemporaneous with man. There are actually legends of a creature which may have been this Glossotherium. "Ancient chroniclers inform us that the indigenous inhabitants recorded the existence of a {182} strange, huge, ugly monster, which had its abode in the Cordillera to the south of latitude 37. The Tehuelches and the Gennakens have mentioned similar animals to me, of whose existence their ancestors had transmitted the remembrance; and in the neighbourhood of Rio Negro, the aged Cacique Sinchel, in 1875, pointed out to me a cave, the supposed lair of one of these monsters, called 'Ellengassen'; but I must add that none of the many Indians with whom I have conversed in Patagonia have ever referred to the actual existence of animals to which we can attribute the skin in question."
A rude painting in a cavern, in red ochre, seems to Dr. Moreno (whose words we have just quoted) to be somewhat suggestive of a Glyptodon. There are some reasons for believing that this quadruped was kept by man as a domestic creature. In the cave are two walls of rough pieces of stone which seem to have dropped down owing to the wearing away of the roof; they also seem to have been loosely piled together to form two walls, within which enclosure an imperfect skull of the animal was found. This skull shows clearly that the so-called "Neomylodon" must be referred to Glossotherium or Grypotherium, as it is sometimes termed. This skull is perforated on the roof in such a way as could only have been effected (in the opinion of experts) by a weapon in the hand of a man. A hole in the skin has been even compared to a bullet-wound. But this it is perhaps unnecessary to discuss. The skin of Glossotherium is, like that of other extinct "Ground-sloths" (e.g. Mylodon), filled with small and irregular ossicles. But in Mylodon, the sculptured appearance of the dermal ossicles appears to indicate that they reached the surface of the body and were covered by epidermis alone, which is not the case with the animal now under consideration. The microscopic characters of the ossicles, too, show differences in the two. Glossotherium being "precisely intermediate between Mylodon and the existing Armadillo (Dasypus)." Now Glossotherium and Mylodon are regarded as forms which lie between the existing Anteaters and the Sloths of the same part of the world. We have already pointed out the facts of structure which lead to this conclusion. It might therefore be reasonably surmised that the hair of Glossotherium would be also intermediate, or at least like that of one of the two genera Myrmecophaga and Bradypus. But microscopical investigation has {183} negatived this supposition. It has shown that the Armadillos are in this matter the nearest relatives of Glossotherium. This result is important as tending further to confirm the close interrelationship of all the American Edentates as contrasted with the Old-World forms--a matter which has already been emphasised. It is suggested, however, that the absence of under fur, which is so well developed in the Sloth, and the difference shown in transverse sections from the hair of Myrmecophaga, may be explained by difference in habitat. Glossotherium lived under conditions similar to those under which the Armadillos live to-day. Thus the outer covering of the body became alike in the two cases, the same needs supervening in both genera.
Lestodon is another allied genus, which seems to possess canines. At any rate, in front of the four molars, and separated from them by a diastema, is a smallish, somewhat canine-like tooth, in both jaws.
Megalonyx and its allies are sometimes placed in a distinct family, MEGALONYCHIDAE. Megalonyx itself had a skull very like that of Bradypus, being shorter and not so elongated as in the Mylodontidae. There is a strong tusk anteriorly, which is separated by a considerable space from the three molars lying behind it. Both pairs of limbs seem to have possessed five toes. This is a North American genus. It differs from the bulk of the American Edentates in having a complete jugal arch.
Megatherium is the type of yet a third family, MEGATHERIIDAE, of the Gravigrade Edentates. This creature is familiar from the many restorations which have been built up, and from its huge bulk, little short of that of an elephant. The skull, which is small for the size of the creature, has a complete jugal arch, from the middle of which depends a downward process as in other allied forms. The teeth grow to an extraordinary depth, and there are five of them in the upper and four in the lower jaw--on each side of course. The fore-limbs of the Megatherium are very much more slender than the enormously bulky hind-limbs, upon which and the equally massive tail the animal seems to have supported itself while tearing down branches of trees, upon whose leaves it fed. In the scapula the acromion joins the coracoid as in Bradypus; the clavicle is large. The fore-limb is four-toed, and the hind-limb three-toed. The latter has but one clawed digit (the third, i.e. the inner). {184} On the manus, the three inner digits have powerful claws. This animal, too, was Pleistocene in time. The Megatheriidae had, however, small as well as gigantic forms.
The genus Zamicrus had a skull no bigger than that of a Sloth, while Nothrotherium was also a comparatively small creature; the teeth of the latter genus are reduced to 4/3.
The extinct group of the GLYPTODONTIDAE comprises large creatures with a dense covering of bony scutes which are arranged in a tesselated fashion, and thus form an immobile armature of immense strength. In correspondence with this massive carapace the dorsal vertebrae have fused together, and the lumbar vertebrae form a series ankylosed to each other and to the following sacrals. These creatures are all South American.
FIG. 106.--Glyptodon clavipes. x 1/12. (After Owen.)
Glyptodon, the genus which gives its name to the family, is known from numerous remains in South America, and also from so far north as Texas and Mexico. It grew to be as long as 16 or 17 feet. In the skull there is an exceedingly long downward process of the zygomatic arch, as in Sloths, the arch itself being complete. The process extends so far down as to reach a point about on a level with the middle of the lower jaw. The nasals are short or rudimentary. As in Myrmecophaga, the pterygoids enter into the formation of the bony palate. The lower jaw has a spout-shaped extremity, and, behind, it rises into an enormous vertical branch as high as the front part of the jaw is long. There are eight teeth in each half of each jaw. As in {185} some Armadillos, the cervical vertebrae are at least partly fused. The atlas is free, but the rest, or at any rate five of them, are united. The last cervical is sometimes fused with the succeeding dorsals; the latter are twelve in number, and are fused together so far as concerns their centra and neural processes. The succeeding region of the vertebral column includes seven to nine lumbars, which are fused with the eight sacrals; in this region the neural processes are high, and there is thus produced a strong and lofty ridge along the back, which forms a powerful support for the carapace. The fore-limbs are shorter than the hind-limbs, which latter are attached to an unusually massive pelvis. The claws of the limbs are blunt and almost hoof-like.
The heavy carapace consists of sculptured, five or six-sided plates, which have no particular arrangement in the middle, but towards the margins show indications of an arrangement in transverse rows. The moderately long tail is also encircled by bony skin-plates which are thorny above, or at least provided each with a blunt upstanding process. It appears that outside this bony system of scutes were horny epidermic scales, corresponding exactly with the tesserae which they cover. There are apparently a good many species of Glyptodon.
In the allied genus Panochthus the tail is rather longer, and the bony rings which surround it, instead of being all movable as in Glyptodon, are at first so, but later, i.e. towards the end of the tail, become welded into a single and massive piece. Both feet are here four-toed, while in Glyptodon the hind-feet are five-toed and the fore-feet four-toed.
Daedicurus shows a further specialisation, in that the feet have three and four digits respectively. The orbit too shows a specialisation in being separated from the temporal fossa. The descending process of the zygomatic arch is not so extraordinarily exaggerated as it is in Glyptodon. It has the same terminal tube of osseous scutes upon the tail. This creature seems to have reached a length of about twelve feet.
Propalaeohoplophorus is, unlike the great Armadillos that we have hitherto dealt with, a small animal, not exceeding 2 feet or so in length of carapace. A small alveolus on each side of the premaxillae seems to suggest the former presence of an incisor tooth; and it seems that the animal possesses both true molars and premolars; for the first four of the eight teeth are much {186} simpler in structure than those which follow. The dorsal vertebrae again are not fused together; the hind-limbs are five-toed. All the plates of the carapace are arranged in definite transverse rows; it has been observed, too, that some of the anterior scutes overlap like those of the Armadillos, to which this animal possesses further likenesses in the exclusion of the maxillae from the border of the nostril (a Glyptodont character), and the comparative feebleness of the scutes.
A primitive genus also appears to be Peltephilus, which is perhaps rather an Armadillo than a Glyptodon. However, it comes somewhat between the two, like Propalaeohoplophorus, with which it may therefore be treated. A most singular feature of this genus has been mentioned on p. 27 in connexion with the skull in the Mammalia generally. That is the fact that a portion of the squamosal surrounding the articular facet for the lower jaw is separated by a suture from the rest of that bone, and is therefore obviously suggestive of the quadrate in the lower Vertebrates. As in certain Armadillos and Glyptodons, etc., the pterygoids appear in this genus to have taken a share in the formation of the hard palate. The plates of the carapace were movable, as is shown by the fact that they sometimes slightly overlap. In view of the possible origin of the Edentates from lowly-organised Mammalia, it is noteworthy that the humerus has been especially compared to that of the Monotreme. Peltephilus differs from other Armadillos in having teeth in the front of the jaws. The total number of teeth is twenty-eight, i.e. seven in each half of each jaw.
SUB-ORDER 2. NOMARTHRA.
As already explained, the Old-World Edentates differ from the New-World forms in having normal dorsal vertebrae, that is to say, without additional zygapophyses. That negative feature, however, though combined with the positive fact that both the Old-World forms feed upon ants, is hardly sufficient to outweigh the many structural differences which distinguish the Orycteropodidae from the Manidae; which will be placed therefore in different groups. To that containing the Aard Vark, the name TUBULIDENTATA may be applied. {187}
This group contains but one family, the ORYCTEROPODIDAE, of which there is but a single genus.
The Aard Vark (earth-pig), genus Orycteropus, is characterised by its heavy build, the body being covered by rather coarse and not very abundant hair; the snout is long and pig-like, with round nostrils at its end; the ears are long, erect, and pointed; the tail is very thick at first, so that it has been aptly described as "a tapering of the body to a point." The fore-limbs are four-toed, the hind five-toed.
FIG. 107.--Aard Vark, or Cape Anteater. Orycteropus capensis. x 1/16.
In the skull there is a complete though slender zygoma; the premaxillaries, though small, are not so rudimentary as in the American Edentates. The annular tympanic is not ankylosed to the surrounding bones, a character found in other low mammals. Contrary to what is found in Manis, Orycteropus has a huge lachrymal. There are thirteen dorsal and seven lumbar vertebrae. The clavicle is well developed. Orycteropus is peculiar among Edentates in that the ischia do not unite with the vertebral column. The femur has a third trochanter.
As mentioned on p. 162, the Aard Vark is diphyodont like normal mammals. The permanent teeth consist of five molars and premolars on each side of each jaw; the first two of these are premolars, and are simpler in their form than the succeeding two teeth, which are partly divided by a median furrow into two halves. These teeth are also peculiar in that they consist entirely of vaso-dentine. They have been compared in minute structure to those of the Ray Myliobates. According to Mr. Oldfield {188} Thomas[115] there are seven milk teeth on each side of the upper jaw (limited to the maxillae, and thus not incisors). An eighth tooth was discovered on one side of one of the specimens examined by Thomas. In the lower jaw there are only four milk teeth on each side. It is interesting to note that the histological structure of these milk teeth agrees with that of the permanent teeth. There are two species of this genus found in Africa: the southern, O. capensis, is more hairy than the northern, O. aethiopicus. O. gaudryi is a Pliocene species from the Island of Samos and from Persia, described by Dr. Forsyth Major and Dr. Andrews.[116] It closely resembles the existing O. aethiopicus.
FIG. 108.--Section of lower jaw with the teeth of Orycteropus. x 2. (After Owen.)
Of the Scaly Anteaters, Group SQUAMATA or MANIDAE, there is really but one genus, though Phatagin, Pholidotus, Smutsia, and Pangolin have been used to distinguish various forms. The genus Manis is African and Oriental in range. Dr. Jentink, who has lately revised the species, allows seven.[117] The external form of these animals is fairly well known, the remarkable scales distinguishing the Pangolins from other animals. Between the scales lie hairs, which seem to be absent in the adults of the African species, though present in the young, thus affording a convenient method of distinguishing the Ethiopian from the Oriental forms. The scales have been compared to agglutinated hairs. That they are not "merely mimetic of the Lizards' scales" is held by Weber,[118] who compares them directly with those {189} structures, as he does the scales of other mammals, such as those upon the tail of Anomalurus, etc. This, however, is not a universal opinion. It is true that these scales occur chiefly in the lower forms of mammals such as those under consideration, Marsupials, Rodents, and Insectivores; but the fact that the hairs are developed before the scales shows, or seems to show, that the former are the older structures, and to lead to the inference that the scales of mammals are new structures. The scattered hairs of the Pangolin have no sebaceous glands excepting on the snout. This, again, looks as if they were degenerate structures, and emphasises the non-archaic character of the scales. These animals have no trace of teeth except possibly some slight epithelial thickenings which have been interpreted as a last remnant; the tongue is suited for the capture of ants, and is therefore much like that of the not nearly-related American Anteaters. The stomach is of simple form; it is characterised by a large gland, which suggests that of the Koala (see p. 144); the intestine has no caecum. Retia mirabilia occur on the limb arteries. The placenta is non-deciduate and diffuse; it is specially compared by Weber with that of the Horse. Considering the many adaptive resemblances between this genus and the American Anteaters, especially in the mouth cavity, it is remarkable that in Manis the pterygoids are not joined as they are in Myrmecophaga. In spite of statements to the contrary, it appears that there is sometimes a distinct lachrymal.
A remarkable feature in the skeleton of Manis is the singular sternum. The xiphoid cartilage is extraordinarily elongated into thin strips, which reach the pelvis and return. This state of affairs is to be found in the African species only. This structure is not comparable, as it has been said to be, with abdominal ribs such as those of the reptile Hatteria.
These animals are mainly anteaters. The Japanese have a curious legend as to the method adopted for the capture of ants, which is related by Dr. Jentink in his monograph of the genus. The Manis "erects his scales and feigns to be dead; the ants creep between the erected scales, after which the anteater again closes its scales and enters the water; he now again erects the scales, the ants are set floating, and are then swallowed by the anteaters"! The same story is related by Mr. Stanley Flower on the authority of the Malays.
Though it seems clear that the likenesses which Manis shows {190} to the Anteaters of the New World are chiefly adaptive and have nothing to do with real affinity, being merely an expression of a similar mode of life, it is curious to note that here and there we do find certain resemblances which do not seem to be susceptible of the latter explanation. The jugal bone, absent in Manis, is small in Myrmecophaga; the clavicle is absent and again small or rudimentary in the Anteaters; it is large in other Edentates. The third trochanter is absent, as in Myrmecophaga (and the Sloths). There are many scales on the body; in Myrmecophaga there are traces of these structures on the tail, as also in Tamandua. In the features mentioned, the Myrmecophagidae differ from either or from both of the two other American families (i.e. Dasypodidae, Bradypodidae) and agree with Manis. The facts are not a little remarkable.
FIG. 109.--Manis. Manis gigantea. x 1/12.
ORDER III. GANODONTA.[119]
Allied to the Edentata, and apparently representing the ancestral forms from which they, at any rate the Xenarthra were derived, is the order of the Ganodonta. Of this order a number of genera are now known, which can be ranged in a series which more and more approaches the Edentata as we pass from the older to the newer forms. This interesting and transitional series will be made manifest by a description of the characters of the various genera taken in their proper {191} chronological order. The following genera are included by Wortman in his family STYLINODONTIDAE.
The earliest type of the Ganodonta is the genus Hemiganus, with but one species, H. otariidens. This animal lived during the deposition of the lowest Eocene strata, the Puerco beds of North America. It was about as big as a fair-sized Dog, and had powerful jaws. There were at least two pairs of incisors in the upper jaw, together with powerful canines and the full premolar and molar formula. In the lower jaw the canines were also strong, but the incisors are not certainly known to be more than two pairs. The enamel upon the posterior surface of the canine is thin, and in the case of the incisors the enamel seems to be limited to the anterior face. The lower molars are quadritubercular. It is believed from the presence of a suture on the upper surface of the premaxillary that the snout of the creature was tubular. The cervical vertebrae, only known by their centra, are like those of the Armadillos (and for the matter of that of the Whales) in the great transverse as opposed to the antero-posterior diameter. The feet are especially compared with those of the Ground Sloths. The single ungual phalanx is marked by a large subungual process, which is pierced by a considerable foramen. The tibia again is to be compared with that of the Armadillos.
In the Upper Puerco (Torrejon) beds the remains of Psittacotherium are found. This genus, when first discovered, was referred to the Tillodontia by some and to the Ungulates, the latter being a refuge for indeterminate Eocene mammals, just as the "Multituberculata" is for similarly-placed Secondary mammals. It is now known to be clearly a member of the order Ganodonta. Wortman thinks that there is but one species, P. multifragum. It seems to have had a general aspect much like that of Hemiganus--that is judging from the skull--and was not very greatly different in size. The facial portion of the skull is short, and the zygoma is deep. The infra-orbital canal is double, a feature which crops up in the Sloth, and has been mentioned in the later form of Ground Sloth, Megalonyx (but it must be remembered that the same characteristic is not unknown in Rodents). The dentition is reduced as compared with that of Hemiganus, that is to say, as far as concerns the molars and the incisors. There is but a single pair of incisors in each jaw; the canines are strong; the premolar and molar series seem to have been complete in the lower jaw, {192} but reduced by one premolar at least in the upper jaw. It is very important to notice that the incisors have enamel only on their anterior faces, and that the same is the case with the canines, the slender layer present behind the tooth in Hemiganus having vanished in this later form. The tooth pattern of the molars is like that of Hemiganus. The fore-limb is decidedly Edentate-like; but it is the foot which presents the strongest likenesses to that order. "If an anatomist," remarks Dr. Wortman, "had no other part of the skeleton than that of the foot to guide his judgment, and he should fail to detect a most striking similarity between it and that of the Edentata, especially the Ground Sloths, he would not only lay himself open to the criticism of being lacking in the ordinary powers of observation and comparison, but would be suspected of placing the matter upon a basis other than that established by such a method." It is not certain how many toes upon the fore-limbs were possessed by Psittacotherium, but the close resemblance to Mylodon is indeed striking, the third digit being in both forms the most pronounced. Some vertebrae of this Ganodont have been discovered which do not show the complex articular arrangements of later American Edentates. The sacrum, on the other hand, is very like that of the Sloth, and there is a foreshadowing of the attachment of the ilia to the sacrum by co-ossification which is met with in later Edentates. A still later type is the genus Calamodon, which has been shown to occur in Europe as well as in America. C. simplex was a larger beast than either of the genera that have already been treated of, thus affording another example of the increase in size of later as compared with earlier members of the same group, so pronounced among the Ungulata. The lower jaw has the same massive structure that characterises that bone in Hemiganus and Psittacotherium. There is but one incisor, but the premolar and molar series are complete. The canine is Rodent-like in appearance, being imbedded throughout the greater part of the lower jaw; it evidently grew from a persistent pulp. It is enamelled upon the anterior face only. The premolar and molar teeth are in this genus commencing to lose their enamel, which is distributed in the form of vertical bands, leaving interspaces which are not covered by enamel. These teeth, moreover, are decidedly hypselodont, more decidedly so than in Psittacotherium; they are, when unworn, quadricuspidate, with accessory cusps; when more worn, the teeth {193} are double-ridged, and that transversely to the long axis of the jaw; finally, the much-worn teeth have flattish crowns more or less surrounded by a ring of enamel.
A still later form, coming from the Lower and Middle Eocene strata, is the genus Stylinodon. S. cylindrifer, which is the more archaic of the two described species, is only known from a single molar, fragments of a canine, and "some inconsiderable pieces of the skull." The molar is interesting on account of the fact that the enamel is still further reduced; it is represented only by narrow vertical strips, which are much narrower than those of older forms of Ganodonts. It is also hypselodont, and has a persistent pulp. So, too, the canine which had a thick anterior facing of enamel. The later species, S. mirus, is more fully known. The teeth seem to have been much the same as in the last-described species; the premolars and molars were seven in all in the lower jaw, and the canine was imbedded in the bone for a long distance, as in Calamodon. The cervical vertebrae have short centra as in Hemiganus. The clavicles were well developed. The humerus possessed an entepicondylar foramen, and its head displays the pyriform pattern so characteristic of later Edentates. The foot is clearly like that of Psittacotherium.
In reviewing the series, therefore, we see a gradual diminution of the incisors, a gradual loss of enamel on the teeth generally, and the production of hypselodont teeth growing from persistent pulps; all of which are features of the later Edentates. The progression is so gradual that the forms enumerated and described seem to have been part of a continuous series culminating in the Ground Sloths of later times. The other points of similarity will be gathered from the facts given in the foregoing pages.
There is another family belonging to the Ganodonta whose position with regard to the Edentata is not so clear. This is the family CONORYCTIDAE, of which two genera are known. The earliest of these, from the Lower Puerco, is Onychodectes. In O. tissonensis the skull is long and narrow, thus contrasting with that of the last family. The facial part is also long. The lower jaw is much more slender. The molar formula was complete, but there is some doubt as to the incisors. The molars are tritubercular.
The other known genus is Conoryctes. Its skull has a shorter {194} facial portion, and is thus more like that of Stylinodontidae than that of Onychodectes. The dental formula is known, and is complete save for the loss of one incisor above and below, and one premolar above. The relationship of these Ganodonts to any later forms is uncertain; but their skeletal structure is as yet by no means fully known.
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