The Osteology of the Reptiles

Front matter

THE OSTEOLOGY OF THE REPTILES

BY

SAMUEL WENDELL WILLISTON

Introduction

PART I

THE SKELETON OF REPTILES

INTRODUCTION

THE PRIMITIVE SKELETON OF REPTILES

That the reptiles were evolved from the Amphibia, and more specifically from that order known as the Temnospondyli, seems now assured. The earliest as also the most primitive reptiles that we know belong to the order called the Cotylosauria. With the exception of Eosauravus from the middle Pennsylvanian of Ohio, of which, unfortunately, the skull is unknown, our knowledge of them goes no further back than the late Carboniferous and early Permian. At that time there was a considerable diversity of known forms, belonging to at least four well-differentiated groups and twenty or more families; from which we may very properly conclude that their earliest ancestors, the beginning of their stock, lived much earlier, certainly at the beginning of the Upper Carboniferous, and very probably in Lower Carboniferous times. We therefore never can expect to find in the rocks of the Permian any real connecting link between the two classes.

Both the reptiles and the amphibians had changed in this interval, an interval perhaps of millions of years, retaining in varying degrees their ancestral characters, while losing or adding others in various ways. The reptiles, by the acquirement of a new mode of life, the loss of gills in their youth and entire emancipation from the water, became more progressive than the amphibians, and their evolution was more rapid. Characters that are common to many amphibians became more and more rare among the reptiles, and the amphibians, handicapped by inherited habits, were restricted more and more to subordinate rôles, and only a few of the more progressive continued to develop. They, for the most part, lost those characters and adaptations that brought them into immediate competition with the reptiles, and by the close of Triassic times had become restricted to habits and habitats no longer invaded by them. The modern toads, frogs, salamanders, and blindworms differ far more from the higher amphibians of Paleozoic times than did the latter from their contemporary reptiles.

Fig. 1. Primitive Cotylosaur Seymouria, from nearly complete specimen, from above. A little less than one third natural size.

Nevertheless, there were still so many inherited characters among both the amphibians and reptiles of early Permian times that nothing distinctive of either class can be found in the skeleton, except in the atlas and feet, with a considerable gap in the structure of their vertebrae. In the vertebral column there was a general change among the Temnospondyli from the embolomerous to the rhachitomous type, that is, from the more simply divided centrum of two disks to the tripartite centrum composed of wedges; while all reptiles had acquired a reduced embolomerous form with one disk, the centrum, and one wedge, the intercentrum.

Doubtless all amphibians of Lower Carboniferous times had embolomerous vertebrae, but only a very few of their stock persisted as late as the Permian. In general literature the Amphibia are distinguished from the Reptilia by the possession of two occipital condyles. The earliest amphibians doubtless all had a single occipital condyle, an inheritance from their ancestral fishes—all that we know from the Lower Carboniferous had—of which only one known descendant with that character survived to the Permian. The reptiles, however, retained the single condyle until the beginning of their evolution into mammals, when they too developed a double condyle. We relied, until recently, upon the widely open palate of the Amphibia as a final distinguishing character of their class, but we now know that some, if not all, of the earliest amphibians had a [closed] palate like that of the [earliest] reptiles, but of these none is known at the beginning of Permian times. In other words, a single condyle and a closed palate are more primitive characters of the tetrapods than those we had assumed as characteristic of the Amphibia. We know no amphibians with as many bones in the digits as the early reptiles possessed, and no reptiles with as many bones in the tarsus as the early amphibians had, but doubtless when we discover the feet of the earliest reptiles we shall find them not different from the feet of the contemporary amphibians.

Every known bone in the skull of the Temnospondyli, except the interfrontal of a very few, has been found in the skull of early reptiles, and all, indeed, in a single genus Seymouria (Figs. 1, 2) from the Lower Permian of Texas. And there is no bone in the skeleton of reptiles that is not known in these same amphibians, except the preparietal of the Anomodontia, the supraorbitals of various Squamata and the predentary and rostral bones of certain dinosaurs, and doubtless the last two, if not all, are simply dermal bones which became temporarily attached to the skull. The girdles and limbs of the two classes are distinguishable only by minor characters. And thus, while we do not know from these later rocks, and probably never shall from rocks later than the Lower Carboniferous, all of the characters common to the two classes in any one animal, from the comparison of all it is not difficult to decide what were the primitive characters of the reptilian skeleton in almost every detail. They may be summarized as follows:

The Primitive Skull of the Reptilia

Rugose, with five openings in roof:

A. Paired, divided, terminal nares.

B. Paired orbits beyond middle [i.e., in front of the middle of the skull].

C. Median parietal (pineal) foramen.

An emargination of the occipital border, between tabular and squamosal, for the ear [the "otic notch"].

Seventeen pairs of roof bones; four pairs of palatal bones; eight pairs of cranial bones; eight pairs of mandibular bones; three unpaired cranial bones; one unpaired palatal bone—seventy-eight in all.

A. Paired Bones

| 1. | Premaxillae (px) | Dentigerous

| 2. | Maxillae (mx)

| 3. | Septomaxillae (sx) | Nasal bones

| 4. | Nasals (na) | Median roof bones

| 5. | Frontals (fr)

| 6. | Parietals (pa)

| 7. | Interparietals (ip)

| 8. | Lacrimals (la) | Surrounding orbits

| 9. | Prefrontals (pr)

| 10. | Postfrontals (pf)

| 11. | Postorbitals (po)

| 12. | Jugals (j)

| 13. | Intertemporals (it) | Temporal bones

| 14. | Supratemporals (st)

| 15. | Tabulars (t)

| 16. | Squamosals (sq)

| 17. | Quadratojugals (qj)

| 18. | Dentaries (d) | Mandible

| 19. | Coronoids (cor)

| 20. | Splenials (sp)

| 21. | Postsplenials (psp)

| 22. | Angulars (an)

| 23. | Prearticulars (pa)

| 24. | Surangulars (sa)

| 25. | Articulars (art)

| 26. | Prevomers (pv) | Palatal bones, dentigerous

| 27. | Palatines (pl)

| 28. | Pterygoids (pt)

| 29. | Ectopterygoids (ec)

| 30. | Quadrates (qu) | Articulation of mandible

| 31. | Exoccipitals (eo) | Cranial bones

| 32. | Paroccipitals (poc)

| 33. | Proötics (pc)

| 34. | Postoptics (al)

| 35. | Stapes (stp)

| 36. | Epipterygoids (ep)

| 37. | Ethmoids (se)

AA. Unpaired Bones

| 38. | Parasphenoid (ps) | Palate

| 39. | Supraoccipital (so) | Cranial bones

| 40. | Basioccipital (bo)

| 41. | Basisphenoid (bs)

The Primitive Postcranial Skeleton

A. Membrane Bones, Exoskeletal

Sclerotic plates in orbits.

Paired clavicles, cleithra and elongate interclavicle.

Dermal plates or scutes.

AA. Cartilage Bones, Endoskeletal

Notochordal vertebrae; two or three cervicals, about twenty-three presacral; one sacral; moderately long tail; proatlas; atlas embolomerous; all vertebrae to tenth or twelfth caudal with free, holocephalous ribs, articulating continuously with intercentrum and diapophysis.

Intercentra between all vertebrae.

Slender and numerous parasternal ribs.

Scapular girdle composed of paired scapulae, procoracoids and metacoracoids, fused in adult life, the three forming the glenoid socket; a supracoracoid and a supraglenoid foramen.

No sternum.

Pelvis plate-like with small obturator foramen only; acetabulum formed by the three bones, closed.

Legs short and stout.

Humerus dilated at extremities, with entepicondylar foramen.

Carpus with four bones in proximal row; two (three?) in middle row; five in distal row; all well ossified.

Hand pentadactylate, the fourth finger strongest and longest; phalangeal formula 2, 3, 4, 5, 3.

Tarsus composed of nine bones: two in first row; two in second; five in distal row; all well ossified.

Feet pentadactylate, the fourth toe strongest and longest; phalangeal formula 2, 3, 4, 5, 4.

Chapter I

CHAPTER I

THE SKULL OF REPTILES

External Appearance, Excrescences, and Chief Openings

The skull of reptiles, as of other vertebrates, has undergone many changes in adaptation to food, offensive and defensive habits. It has lost not a few bones in various forms, and others have united or formed new associations; to such an extent, indeed, that there are several in later reptiles about whose homologies there has been and yet is dispute. It has developed excrescences or horns for defense or offense, or has been covered at times with a solid armor of skin bones; but it has gained permanently no new bones, though a few have been added temporarily from the exoskeleton. The skull of carnivorous reptiles (Figs. 33, 45) is more or less elongate, like that of a wolf; insectivorous reptiles may have a more slender skull (Fig. 52 b); while those reptiles using the jaws to crush invertebrates always have a short and powerful skull (Fig. 49). The face of aquatic, fish-eating reptiles (Fig. 58) is always long, sometimes very long (Fig. 67), as in the modern gavials.

Excrescences or horns on the skull have been developed in not a few. The earliest known is that of the cotylosaurian Chilonyx, with excrescences, and the theromorph Tetraceratops, with large protuberances. Some of the later Cotylosauria, like Elginia, had horny protuberances at the back part. A few carnivorous dinosaurs have a median facial and supraorbital rugosities, as though for the support of horns or spines. In the Ceratopsia (Fig. 70 a) the development of horns and spines was carried to a remarkable degree, not only on the face but also along the posterior margin of the greatly extended skull. Perhaps of all reptiles none has surpassed some of the modern chameleons in the development of facial horns (Fig. 55 d), though not a few other lizards, like the horned lizards and moloch lizards, have many sharp protuberances and horny excrescences, which, were they magnified to the size of dinosaurs would be equally imposing. Even some turtles, like the southern Miolania, have horns upon the skull. Usually the median unpaired facial horn is borne by the nasals, as in the mammals, but in the chameleons it is formed by the maxillae. The paired facial horns are borne by the prefrontals or postorbitals. The frontals and parietals are sometimes developed into enormous crests in the dinosaurs, the supraoccipital in pterosaurs. Doubtless all such horns or protuberances were covered in life with a horny sheath.

The external nostrils (external nares) vary greatly in position. Primitively located near the extremity of the face (Figs. 2, 3, etc.), each was surrounded by the premaxilla, maxilla, nasal and lacrimal, and they almost always have the same relations with the first three of these bones, wherever located. Well separated by the premaxillae and nasals in the older reptiles, they are often confluent in later ones (Figs. 31, 32, 59, 68). They are surrounded by the maxillae in the chameleons (Fig. 55), by the nasals in the phytosaurs (Fig. 66); the nasals are often excluded from them, and the lacrimals have lost all relations with them since Permian times. In most aquatic reptiles they have receded toward the orbits, or rather the face has grown away from them, often for a long distance, as in the ichthyosaurs (Fig. 50), plesiosaurs (Fig. 46 a), proganosaurs, thalattosaurs (Fig. 61), and phytosaurs (Fig. 67). In the very slender-faced amphibious Crocodilia (Fig. 68) and Choristodera (Fig. 63), however, the nostrils retain their primitive position at the extremity of the face. They are located far back from the extremity in the volant pterodactyls (Figs. 71, 72) as in most birds.

The internal nares, or choanae, normally situated almost immediately below the external (Fig. 55), are carried back by a respiratory canal, formed by the undergrowth of the maxillae and palatines as a secondary palate, to a greater or less extent in the Cynodontia and Crocodilia (Fig. 69); in the former and in the early kinds of the latter, to the posterior border of the palatines; in the later crocodiles even into the pterygoids. A similar respiratory canal, probably separated from the cavity of the mouth by a membrane only, is characteristic of the Phytosauria (Figs. 66, 67). A partial secondary palate, formed by the union of the palatines or maxillae, with the opening only a little way back, occurs in some Chelonia and Anomodontia. In those reptiles in which the external nares are situated posteriorly, the internal nares are also (e.g., Figs. 61, 66). In the plesiosaurs only (Fig. 46), there may be a partial reversion of the respiratory canal, with the internal opening in front of the external. The internal nares, primitively (Figs. 6, 21, 47) divided by the prevomers and surrounded by the premaxillae, maxillae and palatines, may sometimes (Figs. 49, 71 c) lie between the prevomers and palatines.

The parietal or pineal foramen, very large in certain shell-eating cotylosaurs (Fig. 22), had become inconstant even in that order. It is present, so far as known, in all the Theromorpha (Figs. 33–42), and in the Therapsida (Figs. 43–45), with the exception of a few forms; in the Proganosauria, Ichthyosauria (Fig. 50), Sauropterygia (Fig. 48), the Diaptosauria (Figs. 60–62), and most lizards (Figs. 55 a, 56), but is absent in some true lizards, the chameleons, and all snakes (Fig. 59). It has been reported in certain doubtful Pseudosuchia and more or less doubtfully in a few phytosaurs and dinosaurs, but with these possible exceptions appears to be absent in all the Archosauria (Figs. 65 b, 66 b, 68, etc.) as also the Chelonia (Figs. 30, 31, 32). Usually located between the parietals anteriorly (Figs. 22, 33, 43, 44, 45, 46, etc.), it may occur between the frontals posteriorly (Fig. 55). In the Anomodontia and Gorgonopsia (Fig. 43) there is a separate bone, the preparietal, a small unpaired element of doubtful homologies, absent in other reptiles, in front of or surrounding the foramen.

The orbits, directed upward sometimes in aquatic animals (Fig. 32), but usually laterally, were primitively (Figs. 2, 3, 22, 23, 33, 43, 44, 65) surrounded by the prefrontal, postfrontal, postorbital, jugal, and lacrimal. The frontal usually forms a part of the upper margin, the maxillae sometimes below (Figs. 30, 48, 49, 55, 56, 59). In snakes (Fig. 59), only the prefrontal and postorbital may be left. Sometimes the postorbital bar is incomplete in lizards (Fig. 56), snakes (Fig. 59), and therapsids (Fig. 45 d). The prefrontal is excluded in some dinosaurs, the postorbital in Araeoscelis (Fig. 52) and Hyperodapedon (Fig. 62 d), leaving not a single element invariably associated with the orbit. Antorbital or preorbital vacuities are very characteristic of the Archosauria, occurring in all phytosaurs (Figs. 66 b, 67 a) and true pseudosuchians (Fig. 65 b, d), most Saurischia (Fig. 70 a, b) and Pterosauria (Fig. 71) and some Crocodilia. Usually there is but one, but there may be two or even three on each side in certain Theropoda (Fig. 70 a).

Openings through the skull roof, back of the orbits, are characteristic of all reptiles save the Cotylosauria (Figs. 2, 4, 19, 22) and Chelonia. The upper opening, the supratemporal, arose primitively by the separation of the postorbito-squamosal bar (Fig. 33 a) from the parietal (Fig. 53 c). The lower or lateral temporal opening appeared primitively (Figs. 33, 53 a) between the squamosal and the jugal. It is bounded above by the postorbito-squamosal arch, below by the jugal, to which was added, in some of the double-arched forms, the quadratojugal (Figs. 62, 64, 65, 70 a, b). Either the upper or the lower opening may occur independently, or both together. In the Cynodontia (Fig. 45) and some other Theriodontia, with a lower temporal opening (Figs. 44 f, 45 d), the squamosal may fail to meet the postorbital above the opening, permitting the parietal to form the upper boundary in part; and this is the condition in mammals. In not a few of the Therapsida, the Dinocephalia especially (Fig. 44 b), the jugal is excluded from the lower margin by the union of the squamosal and postorbital.

All known forms of the Sauropterygia (Figs. 46, 47, 48) and Placodontia (Fig. 49) have the single opening bounded below by the squamosal and postorbital, above by the sides of the parietal, that is, it is like the upper one of those reptiles with two temporal openings. It is usually considered to be what it really appears to be, the upper temporal opening only; and its certain nature will not be determined until more is known of their terrestrial antecedents.

The intertemporal vacuity. The single temporal opening of the Squamata (Figs. 55, 54), when complete, the Ichthyosauria (Fig. 50), and certain other forms here grouped under the Parapsida, lies between the postorbito-squamosal arch and the parietal, but has, in most if not all, an additional bone helping to form its posterior or outer border, the supratemporal or tabular, for there is doubt as to its real homology. (See pages 61–69 below.)

The post-temporal opening is situated on the occipital aspect of the skull, a vacuity between the parietal, or parietal and squamosal, and the paroccipital on each side. It is present in some Cotylosauria (Fig. 21 b), and Theromorpha (Fig. 45 g), and is generally present in later reptiles though absent or vestigial in the Crocodilia.

The cranial region thus exposed by these various openings has been exposed to a greater or less degree in most Chelonia (Figs. 30–32) in a different way: by the emargination of the roof bones from behind or from behind and below, until, in some forms like the terepenes, the whole temporal roof is lost.

Posterior palatine or suborbital openings occur in most reptiles (Figs. 55, 63, 66, 69, 72) since the Theromorpha, but are absent in some turtles. They are situated between the palatines and maxillae posteriorly, and are usually also bounded in part by the ectopterygoids. They do not occur in the Cotylosauria (Figs. 6, 21 a, 24, 29) or Theromorpha (Figs. 40 c, 42 c), though present in many Therapsida.

The Skull Elements

The primitive relations of the skull elements may be discussed seriatim, with their chief modifications in later reptiles.

Premaxillae (px). Primitively short (Figs. 2, 3, 4, 19, 22, 33, 43), articulating with maxillae, nasals and prevomers, the posterior process forming a partial division between the nasal chambers. They form the anterior boundary of the external and internal nares. Four or five teeth in each.

Elongate in the strictly aquatic reptiles (Figs. 46, 47) and in the Pterosauria. In the plesiosaurs (Figs. 46, 47), pterosaurs (Fig. 72 a), some lizards (Fig. 56), and thalattosaurs (Fig. 61), a median prolongation separates the nasals, articulating directly with the frontals, in the first group (Fig. 46) sometimes directly with the parietals, separating the frontals. Edentulous in the chameleon lizards (Fig. 55), they take no part in the boundary of the nares. They are also edentulous in the turtles (Figs. 30–32), anomodonts (Fig. 44 c), some dromasaurians, the later pterodactyls (Fig. 72), most ornithischians (Fig. 70 c, d), the chameleon lizards, and many snakes. Teeth, when present, are in a single row and rarely exceed five or six in number in each, though there are as many as twenty-three in some phytosaurs (Fig. 66) and even more in ichthyosaurs, where the dentigerous border is greatly elongated. The dentigerous part is short in the long-faced plesiosaurs (Fig. 48 c). They are often fused (Fig. 72), and sometimes united with the nasals (Fig. 54 c).

Maxillae (mx). Primitively (Figs. 2 b, 3, 4, 6, 19, 21, 22, 33, 43), articulating anteriorly with premaxillae, above with septomaxillae and lacrimals, posteriorly with the jugals, ectopterygoids, and postorbitals internally, forming the outer boundary of both external and internal nares in part.

In most reptiles since Permian times they also articulate with the nasals above (e.g., Figs. 33, 43, 44); in the crocodiles (Fig. 69 b) with each other on the palate, as also in many Anomodontia and Theriodontia. They are edentulous in the Chelonia (Figs. 30–32), later Pterosauria (Fig. 72), some Anomodontia (Fig. 44 c), Dromosauria, Ophidia, Saphaeosaurus, Ornithomimus, etc. The teeth may be in single or numerous rows.

Fig. 3. Pantylus (Cotylosauria), from side. Three fourths natural size.

Septomaxillae (sx). Small bones, the so-called turbinals of reptiles, located partly within the nasal chamber, but appearing more or less on the outer side at the back part of the external nares. (Figs. 33, 44 a, b, f, 45.) Present probably in all the earliest and most early reptiles, and in most Squamata (Fig. 59); they are absent in the Chelonia (Figs. 28–32) and Crocodilia (Figs. 68, 69). In some Dromasauria they extend back on the face to meet the lacrimals. Little can be said about them in other extinct reptiles.

Nasals (na). Articulating with premaxillae, frontals, prefrontals, and lacrimals, forming more or less of the partition between the external nares.

Except in most Cotylosauria (Figs. 2, 3, 22, 23, etc.), some Theromorpha and Therapsida, they also articulate with the maxillae on the sides. They are absent in many Chelonia (Figs. 28–32); either absent or fused with the premaxillae in the Mosasauria (Fig. 54 c); separated by the premaxillae in the plesiosaurs, pterosaurs (Fig. 72), and some lizards (Fig. 56), and probably absent in some of the former (Fig. 46). They do not enter into the formation of the nares in the Rhiptoglossa (Fig. 55 d), but surround them in the Phytosauria (Figs. 66, 67). Very large in the Ichthyosauria (Fig. 50), they also articulate with the large postfrontals. They are often fused in the midline.

Frontals (fr). Primitively (Figs. 2, 4, 22, 23, etc.) articulating with nasals, prefrontals, postfrontals, parietals, and ethmoids, often forming the middle of the upper margin of the orbits.

Always present and not varying much in their relations. In the plesiosaurs (Fig. 46), pterodactyls (Fig. 72), and some lizards, they articulate directly with the premaxillae (Fig. 56), and in some of the former are separated externally in the middle. Often fused in midline (Figs. 68, 69).

Parietals (pa). Primitively (Figs. 2, 4) articulating with frontals, postfrontals, intertemporal, supratemporals, tabulars, and interparietals; below with the supraoccipital, epipterygoids, postoptics, and proötics.

In the absence (e.g., Figs. 31 b, 33, 44, 45, etc.) of the intertemporal and supratemporals, the parietals articulate directly with the squamosals and postorbitals; in the Chelonia (Fig. 30) and Crocodilia, also directly with the pterygoids. Fused in most late reptiles (e.g., Figs. 69, 72).

Interparietals (ip). Primitively (Figs. 2 a, 4, 22) back of the parietals on the superior surface of the skull, articulating with parietals, tabulars, and supraoccipital.

In the later Cotylosauria (Fig. 9), most if not all Theromorpha (Figs. 33, 42 d), some Therapsida (Figs. 44 a, d, g), they are situated on the occipital surface and are usually unpaired. A vestige, supposed to be these bones, occurs in some Crocodilia, originally named dermosupraoccipitals. They do not help form any part of the cerebral wall. Unknown or doubtful in other reptiles.

Lacrimals (la). Primitively (Figs. 2 b, 3, 22, 23, etc.) large, extending from orbit to nares, articulating with prefrontals, nasals, septomaxillae, maxillae, and jugals.

In the latest Cotylosauria (Procolophon), most Theromorpha (Fig. 33) and Therapsida (Figs. 43, 44, 45), and all other reptiles, they are excluded from the narial margin. They are small or vestigial in the Squamata, and absent in most Chelonia and in Sphenodon (Fig. 60 a). They are of extraordinary size in some Theropoda (Fig. 70 a), articulating posteriorly with the postorbitals. It has been urged by Jaekel and Gaupp that these bones are not the homologues of the mammalian lacrimal, and should be called by another name, for which postnasal and adlacrimal have been proposed.

Fig. 4. Pantylus, from above. Three fourths natural size.

Prefrontals (pr). Primitively (Figs. 2, 4, 22) at the upper anterior border of the orbits, articulating with lacrimals, nasals, frontals, and postfrontals, and by a descending process with the palatines.

Never absent, though much reduced and excluded from the orbital margin in the Theropoda (Fig. 70 a). Sometimes (Fig. 70 c) they articulate with the postorbitals or postfronto-orbitals when the postfrontals are absent as discrete bones. Below, they articulate with the prevomers in the Chelonia (Fig. 30 b), with the palatines and pterygoids in the Crocodilia (Fig. 69 d). Excluded from the frontals in the Ichthyosauria (Fig. 50). Thought by some to be the homologues of the mammahan lacrimals, and so called.

Postfrontals (pf). Primitively (Figs. 2, 4, 22) at the upper posterior border of the orbits, articulating with prefrontals, frontals, parietals, the intertemporal or supratemporals when present, and with the postorbitals.

In the Chelonia (Figs. 30 a, 31 b, c, 32 a), Crocodilia (Figs. 68, 69), many mosasaurs (Fig. 54), lizards (Fig. 56) and the snakes (Fig. 59), the Pterosauria (Figs. 71, 72), Dinosauria (Fig. 70), and many Therapsida (Figs. 44 d, 45), they are absent or fused with the postorbitals which take their place. Sometimes they (Fig. 46 c, 49 a) help form the anterior boundary of the upper temporal opening. They extend forward to the nasals in the Ichthyosauria (Fig. 50).

Postorbitals (po). Primitively (Figs. 2, 3, 4, 5, 22) at the upper back part of the orbits, articulating with postfrontals, jugals, and squamosals. A descending process also articulates with the maxillae or ectopterygoids.

In the absence of the intertemporal and supratemporal, the postorbitalalso articulates with the parietal (Figs.28, 30, 33, 43). In the absence of the postfrontal it takes its place, often extending forward to meet the prefrontal (Figs. 30, 45 b), or even the lacrimal (Fig. 70 a) in the Theropoda. It still retains its connection with the maxillae [sic] in certain Chelonia (Fig. 31 b) and most snakes (Fig. 59), but not in most other reptiles. Rarely in the lizards (Fig. 56) it does not meet the squamosal. In the Crocodilia (Fig. 69) it is large, and may articulate with frontal, parietal, jugal, quadratojugal, and squamosal. It extends far back in the Chelonia (Figs. 30, 31 a), forming a large part of the temporal roof, articulating with the quadratojugal, the latter also in the Ichthyosauria (Fig. 50). It is extensive also in some of the Dinosauria (Fig. 70), supporting the paired horns of the Ceratopsia (Fig. 70 d).

Jugals (ju). Primitively (Figs. 2 b, 3, 5, 22, 33) large, forming the under boundary of the orbits, articulating in front with lacrimals and maxillae, above with postorbitals, and, by an extensive overlapping suture, with the squamosals and quadrate jugals; on the inner side perhaps with the ectopterygoids.

Absent in some Ophidia (Fig. 59 b) and some lizards (Fig. 56). In the lizards (Fig. 55) they may not articulate with the squamosals.

Intertemporals (it). An amphibian bone known only in Seymouria (Figs. 2, 19) of the Cotylosauria, intercalated between the postfrontal, parietal, supratemporal, and postorbital.

Supratemporals (st). Primitively (Figs. 2, 4, 19) articulating at the sides of the parietals with the postfrontal and postorbital anteriorly, the tabulars behind, and the squamosals on the outer side; interrupted by the otic notch in Seymouria (Figs. 2, 19).

Absent in the more specialized Cotylosauria, probably all Theromorpha, and all other reptiles save possibly the Ichthyosauria (p. 62) and Squamata (p. 65). Generally known as the posterior bone of the arch in the Squamata (Fig. 55 a, ta). [But see tabular below.]

Tabulars (t). Primitively (Figs. 2, 4, 22) on the dorsal surface of the skull in the Cotylosauria, as in the Amphibia, at the outer side of the interparietals, articulating with the squamosals and supratemporals, with the upper end of the quadrates and the outer end of the paroccipitals, whence the name "paroccipital plates" given to them by Baur. They are known to be absent in but a single genus of Cotylosauria; are probably present in most Theromorpha (Figs. 33 b, 42 d) and Therapsida (Fig. 44 g), and some "Pseudosuchia" (Fig. 65 c). The tabular has been identified by the author as the posterior bone of the arch in the Squamata (p. 62), and Ichthyosauria (p. 62), usually and perhaps correctly called the supratemporal. It is unknown in other reptiles.

Squamosals (sq). Primitively (Figs. 2, 4) articulating with tabulars and supratemporals above, in the absence of the latter directly with the parietals (Figs. 33 a, 53 a). Below, they cover the posterior part of the temporal region, extending back of the quadrate to articulate with the pterygoids (Fig. 7), overlapping extensively the quadratojugals on the sides (Fig. 33 a), and articulating in front with jugals and postorbitals.

In later reptiles the squamosal has undergone many changes, but is always present, though sometimes vestigial in the Chelonia, Squamata, and Ichthyosauria. Only in the Cotylosauria and some Theromorpha does it articulate with the pterygoids. In later forms it articulates with the paroccipital to a limited extent, supporting the head of the quadrate. In many Therapsida (Figs. 43, 44 b), but not in the more primitive Theromorpha (Fig. 33 a), it may articulate with the postorbital below as well as above the temporal opening. Its relations with the quadratojugal are also inconstant, lost in the Crocodilia (Fig. 69 c) and Predentata (Fig. 70 c). On the other hand, it may extend forward to unite with the maxillae in some plesiosaurs (Fig. 46 b). In the Squamata (Fig. 55 a), as most recent authors identify the squamosal, it articulates with the bones usually called the supratemporal and the postorbital (rarely excluded from it) and usually with the jugal.

Quadratojugals (qj). At the outer posterior side of the temporal region (Figs. 2 b, 3, 22, 33), overlapped by the squamosals, articulating in front narrowly with the jugals, behind with the quadrates.

The quadratojugals are relatively large in the primitive skull, sometimes forming a part of the articular surface for the mandible (Fig. 21 b). In the single-arched skull the quadratojugal tends to disappear. It is probably present in all Theromorpha, but is often confined chiefly to the posterior side of the quadrate (Fig. 42 d). It is absent in most Therapsida, the Sauropterygia and the Squamata. It enters into the boundary of the lower temporal opening only in the Crocodilia (Fig. 69), Phytosauria (Fig. 66 b), Pseudosuchia (Fig. 65), Pterosauria, Theropoda (Fig. 70 a), and some Predentata (Fig. 70 d), excluded in many Predentata (Fig. 70 c), as in all the other double-arched reptiles. It is very large in some Chelonia (Fig. 30 a), articulating with the postorbitals, as is also the case in the Crocodilia (Fig. 69 c).