EXOSKELETON.

The exoskeleton both epidermal and dermal is exceedingly well developed in reptiles.

Epidermal Exoskeleton.

This generally has the form of overlapping horny scales which invest outgrowths of the dermis, and are found covering the whole body in most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In the Ophidia the ventral surface of the tail is commonly covered by a double row of broad scales, while the ventral surface of the precaudal part of the body is covered by a single row. In the burrowing snakes (Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do not occur, the scales of the ventral surface being similar to those of the dorsal.

In the Chelonia with the exception of Dermochelys, Trionyx and their allies there is a well-developed system of horny shields having a regular arrangement which has been described in the account of the Turtle's skeleton[91].

The rattle of the rattlesnake is an epidermal structure formed of several loosely articulated horny rings, produced by the modification of the epidermal covering of the end of the tail, which instead of being cast off when the rest of the outer skin is shed is retained loosely interlocked with the adjoining ring or joint. New rings are thus periodically added to the base of the rattle, and in old animals the terminal ones wear away and are lost.

Horny claws occur on the ends of some or all of the digits in most living reptiles.

Owen's Chameleon bears three epidermal horns, one arising from the nasal and two from the frontal region.

In the Chelonia, some of the Theromorpha such as Udenodon and Dicynodon, probably also in the Pterosauria and Polyonax among the Dinosaurs, the jaws are more or less cased in horny beaks. The horny beaks of Chelonia are variable; sometimes they have cutting edges, sometimes they are denticulated, sometimes they are adapted for crushing.

Dermal Exoskeleton.

Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and Pythonomorpha, and some Lacertilia such as Tiliqua, Scincus and Anguis have a dermal exoskeleton of bony scutes, developed below and corresponding in shape to the epidermal scales. Sometimes as in Caiman sclerops, Jacare and Teleosaurus, the scutes completely invest the body, being so arranged as to form a dorsal and a ventral shield, and a continuous series of rings round the tail. In Crocodilus they are confined to the dorsal surface, and in Alligator to the dorsal and ventral surfaces. The scutes of some extinct forms articulate with one another by a peg and socket arrangement as in some Ganoid fish.

The carapace of most Chelonia is a compound structure, being partly endoskeletal and formed from the ribs and vertebrae, partly from plates derived from the dermal exoskeleton. The common arrangement is seen in fig. 36. All the surface plates are probably exoskeletal in origin, but united with the ventral surfaces of the costal and neural plates respectively are the expanded ribs and neural arches of the vertebrae.

The plastron in the common genus Chelone (fig. 37) includes nine plates of bone, one unpaired and four pairs; they will be referred to in connection with the ribs and pectoral girdle.

In the Leathery Turtle (Dermochelys) the carapace and plastron differ completely from those of any other living form. The carapace consists of a number of polygonal ossifications fitting closely together and altogether distinct from the vertebrae and ribs. The plastron is imperfectly ossified, and not united with the pelvis, and the whole surface of both carapace and plastron is covered with a tough leathery skin, without horny shields.

Some of the extinct Dinosauria have an enormously developed dermal exoskeleton. Thus in Stegosaurus and Omosaurus the dorsal surface is provided with flattened plates or with spines reaching a length of upwards of two feet. In Polacanthus the posterior part of the body is protected by a bony shield somewhat recalling that of the little armadillo Chlamydophorus. No exoskeleton is known in Ichthyosauria, Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, and some Lacertilia, such as Chamaeleon and Amphisbaena.

Teeth.

The teeth of reptiles are generally well developed, and in the great majority of forms are simple conical structures, uniform in character, generally somewhat recurved, and often with serrated edges. Another common type of tooth is that with a laterally compressed triangular crown provided with a double cutting edge which may or may not be serrated. The teeth are mainly formed of dentine, with usually an external layer of enamel, and often a coating of cement on the root. Vasodentine is found below the dentine in Iguanodon. The teeth of reptiles never have the enamel deeply infolded, nor do they have double roots.

Teeth may be present not only on the jaw-bones, but as in many Squamata, also on the palatines, pterygoids or vomers. The method by which they are attached to the bones varies much. Sometimes as in Iguana and some other lizards, they are pleurodont[1], sometimes they are acrodont[92], as in the Rhynchocephalia, Pythonomorpha, Ophidia and some Lacertilia such as Agama. Again they may be set in a continuous groove as in the Ichthyosauria and young Crocodilia. Finally the teeth may be thecodont or placed in distinct sockets as in the Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and Theropoda. In Iguanodon the teeth are set in shallow sockets in a groove one side of which is higher than the other; the method of attachment thus shows points of resemblance to the thecodont condition, the pleurodont condition, and that met with in the Ichthyosauria.

In Ichthyosaurus the teeth are marked by a number of vertical furrows, and it is from a furrow of this nature greatly enlarged and converted into a tube that the channel down which flows the poison of venomous snakes is derived.

In most reptiles the dentition is more or less homodont. The only reptiles in which a definite heterodont dentition is known are the extinct Theromorpha, and in them the teeth vary greatly. Thus Udenodon is toothless, the jaws having been probably cased in a horny beak. In Dicynodon the jaws are likewise toothless with the exception of a pair of permanently growing tusks borne by the maxillae. Dicynodon is the only known reptile whose teeth have permanently growing pulps. In Pariasaurus the teeth are uniform and very numerous, and though placed in distinct sockets are ankylosed to the jaw. In Galesaurus and Cynognathus three kinds of teeth can be distinguished, slender conical incisor-like teeth, large canine-like teeth, and cheek teeth with two or three cusps. The teeth in Galesaurus are confined to the jaws, in Placodus and its allies, however, large flat crushing teeth are attached to the palatines as well as to the jaw-bones, and in Pariasaurus the vomer, palatine and pterygoid all bear teeth as well as the jaw bones. The upper jaw of Sphenodon and other Rhynchocephalia is provided with two parallel rows of teeth, one borne on the maxillae and one on the palatines, the mandibular teeth bite in a groove between these two rows. The bone of the jaws in Sphenodon is so hard that when the teeth get worn away, it can act as a substitute. In the young Sphenodon the vomers bear teeth, as they do also in Proterosaurus.

There is generally a continuous succession of teeth throughout life, the new tooth coming up below, or partly at the side of the one in use, and causing the absorption of part of its wall or base. In this way the new tooth comes to lie in the pulp cavity of the old one. This method of succession is well seen in the Crocodilia.

Teeth have been detected in embryos of Trionyx, but otherwise no teeth are known to occur in Chelonia, or in Pteranodon (Pterosauria), while the anterior part of the jaw is edentulous in Iguanodon, Polyonax and some other Dinosaurs, and in Rhamphorhynchus.

ENDOSKELETON.

Vertebral column.

The vertebral column is commonly divisible into the usual five regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among Lacertilia, only into caudal and precaudal regions. In the Chelonia there are no lumbar vertebrae.

The form of the vertebral centra is very variable. A large proportion of extinct reptiles,—several entire orders,—and the earlier and more primitive forms in some of the other groups have amphicoelous vertebrae. Vertebrae of this type occur in the Theromorpha, Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many Dinosauria, also in some of the early Crocodilia such as Belodon, Teleosaurus and Goniopholis, and the Geckonidae among Lacertilia.

The majority of living reptiles have procoelous vertebrae. Thus they occur in the Lacertilia (excluding the Geckos), the Ophidia, and the Crocodilia, also among extinct forms in the Pterosauria and many Dinosauria. On the other hand some Dinosauria such as Iguanodon have opisthocoelous cervical vertebrae, while others have opisthocoelous thoracic vertebrae. The vertebrae of the Ceratopsidae and some Sauropterygia, the thoracic vertebrae of Iguanodon, and the sacral vertebrae of Crocodilia have flat centra. The first caudal vertebra of modern Crocodilia is biconvex, and in the Chelonia all types of vertebral centra are found. The cervical vertebrae of Sphenodon are noticeable for the occurrence of a small pro-atlas, which may represent the neural arch of a vertebra in front of the atlas.

In most reptiles the vertebrae are fully ossified, but in some of the more primitive forms the notochord persists in the centre of the vertebra (i.e. intervertebrally), this is the case for instance in many of the Theromorpha and Rhynchocephalia, and also in the Geckos. In other reptiles it persists longest intravertebrally.

The centrum of each of the caudal vertebrae of most Lacertilia is traversed by an unossified septum along which it readily breaks.

Chevron bones occur below the caudal vertebrae in Lacertilia, Chelonia, Ichthyosauria, many Dinosauria, and Sphenodon, articulating with quite the posterior part of the centrum which bears them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a well-marked odontoid process. The ventral portions of the intervertebral discs are sometimes ossified, forming wedge-shaped inter centra, as in Geckos, and the cervical vertebrae of Sphenodon.

In snakes, Theropod Dinosaurs, and the iguanas among lizards, the neural arches are provided with zygosphenes, and zygantra.

The neural arches are usually firmly ankylosed to the centra, but in the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the suture between the centrum and neural arch persists at any rate till late in life. In the Ichthyosauria the neural arches were united to the centra by cartilage only.

The thoracic vertebrae of some of the Theromorpha (Dimetrodon) are remarkable for the extraordinary development of the neural spine, and those of Chelonia for the absence of transverse processes.

In living reptiles the number of sacral vertebrae is nearly always two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as five or six bones may be ankylosed together in the sacral region. In Crocodiles the two halves of the pelvis sometimes articulate with different vertebrae. The vertebrae of some of the great Sauropoda are remarkably hollowed out, having a large vacuity on each side of the centrum communicating with a series of internal cavities. The whole structure of these vertebrae shows a combination of great strength with lightness.

The Skull.

The reptilian skull is well ossified and the bones are noticeable for their density. The true cranium is often largely concealed by a secondary or false roof of membrane bones, which is best seen in the Ichthyosauria and some of the Chelonia. In other reptiles the false roof is more or less broken up by vacuities exposing the true cranial walls. The ethmoidal region is the only one in which much of the primordial cartilaginous cranium remains. The lateral parts of the sphenoidal region are also as a rule not well ossified.

In some reptiles, such as most Lacertilia and Chelonia, the orbits are separated only by the imperfect interorbital septum, while in others, such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity extends forwards between the orbits.

In the occipital region all four bones are ossified. The great majority of reptiles have a single convex occipital condyle, but some of the Theromorpha such as Cynognathus have two distinct condyles as in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the exoccipitals, as well as the basi-occipital, take part in the formation of the single condyle; sometimes, as in Crocodiles, it is formed by the basi-occipital alone, as in birds. The relations of the bones to the foramen magnum vary considerably, in Chelonia the basi-occipital generally takes no part in bounding it, and in the Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded. The parietals are paired in Geckos and Chelonia alone among living forms, and in the extinct Ichthyosauria and some Theromorpha; in all other reptiles they are united.

The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia, Ophidia, Sphenodon (fig. 52, B, 4) and some extinct crocodiles, such as Belodon. They are completely fused in living Crocodilia and some Lacertilia and Dinosauria. In the gigantic Polyonax they are drawn out into a pair of enormous horns, and the parietals and squamosals are greatly expanded behind.

An interparietal foramen occurs in the Theromorpha, the Ichthyosauria (fig. 32, 10), Sphenodon, the Sauropterygia and most Lacertilia. The posterior part of the skull is curiously modified in some Chamaeleons, the parietals and supra-occipitals being drawn out into a backwardly-projecting sagittal crest which unites with the two prolongations from the squamosals. In other Chamaeleons (C. bifidus) prolongations of the prefrontals and maxillae form large forwardly-projecting bony processes.

The roof of the skull is characterised by the development of prefrontals and postfrontals, which lie respectively near the anterior and posterior extremity of the orbit. In Theromorpha, Squamata, Crocodilia, and some Dinosauria lachrymals are developed. There is a ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the Metriorhynchidae among Crocodiles and some Rhynchocephalia, Dinosauria, and Pterosauria.

The pro-otic lies in front of the exoccipital and together with the opisthotic forms the hind border of the fenestra ovalis. In Chelonia the opisthotic remains separate, in all other living reptiles it fuses with the exoccipital. The epi-otic fuses with the supra-occipital.

The parasphenoid, so important in Ichthyopsids, has very often disappeared completely; it is present, however, in the Ichthyosauria, the Plesiosauridae, and a number of Squamata, in many Ophidia its anterior part forming the base of the interorbital septum.

In the Plesiosauridae and most Lacertilia, but not in the Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the parietal or the anterior end of the pro-otic with the pterygoid. A homologous arrangement occurs in the Ichthyosauria and some Chelonia.

In most reptiles a transpalatine occurs, connecting the maxillae with the pterygoid, but this is absent in the Chelonia, and some Dinosauria, and in the Typhlopidae among snakes.

The quadrate is always well developed, and except in the Squamata is firmly fixed to the surrounding bones. The Chamaeleons also, among the Squamata, have a fixed quadrate, and in them too the quadratojugal is absent. Separate nasal bones do not occur in any living Chelonia.

The vomers are generally paired as in Squamata, sometimes unpaired as in Chelonia.

The disposition of the bones of the jaws is subject to much modification in the Ophidia in order to adapt them for swallowing very large prey. The arrangements again differ greatly in the venomous and non-venomous snakes. In the non-venomous snakes, such as Python and Tropidonotus, the palatine is large and is fixed to the pterygoid which extends outwards (fig. 51, 10) so as to be united to the quadrate, and is at the same time firmly connected by the transpalatine with the maxillae. The quadrate is united to the squamosal, which is loosely attached to the cranium. The premaxillae is moderately developed and bears teeth, and the maxillae forms a long bar loosely connected with the rest of the skull. The rami of the mandible are united only by an extremely elastic ligament. It is as regards the maxillae and premaxillae that the skulls of venomous and non-venomous snakes differ most. In the rattlesnake (Crotalus) and other venomous snakes the premaxillae is extremely small and toothless. The maxillae is small and subcylindrical, and is movably articulated to the lachrymal, which also is capable of a certain amount of motion on the frontal. The maxillae is connected by means of the transpalatine with the pterygoid, which in its turn is united to the quadrate. When the mouth is shut the quadrate is directed backwards, and carrying back the pterygoid and transpalatine pulls at the maxillae and causes its palatal face, to which the poison teeth are attached, to lie back along the roof of the mouth. When the mouth opens the distal end of the quadrate is thrust forward, and this necessitates the pushing forward of the pterygoid and transpalatine, causing the tooth-bearing surface of the maxillae to look downwards and the tooth to come into the position for striking.

The Ophidian skull is also noticeable for the absence of the jugals and quadratojugals. In poisonous snakes the place of the jugal is taken by the zygomatic ligament which connects the quadrate and maxillae.

The extent to which the palate is closed in reptiles varies much. In many reptiles, such as the Squamata and Ichthyosauria, the palate is not complete, both palatines and pterygoids being widely separated in the middle line. In others, such as the Crocodilia, Sauropterygia, and Chelonia, there is a more or less complete bony palate. In many Chelonia this is chiefly formed of the vomer, palatines, and pterygoids, the posterior nares being mainly bounded by the palatines. In living Crocodilia, however, outgrowths are formed from the pterygoids and palatines which arch round and meet one another ventrally, forming a secondary palate (fig. 43, A), which completely shuts off the true sphenoidal floor of the skull, and causes the posterior nares which are bounded by the pterygoids to open very far back. Though this feature is common to all postsecondary crocodiles, it is interesting to notice that it is not found in the earlier forms, but that its gradual evolution can be traced. In the Triassic Belodon, for instance, the posterior nares open far forwards, and are not surrounded by either the palatines or pterygoids. In the Jurassic crocodile, Teleosaurus, the posterior nares lie further back, being surrounded by the palatines, but the pterygoids do not meet them. Finally, in the Tertiary forms the arrangements are as in living crocodiles.

A short secondary hard palate is found also in the Theriodontia. The palatines of Ichthyosaurus are noticeable for their transverse position, which recalls that in the Frog.

The various fossae or vacuities in the false roof of the skull are important, and their relations may best be understood by a description of their mode of occurrence in Sphenodon, a form in which they are very completely developed.

In Sphenodon, then, on the dorsal surface of the skull, are the large supratemporal fossae (fig. 52, 20). Their inner margins are separated from one another by the parietal walls of the cranium, while externally each is bounded by a bony arch, the supratemporal arcade, formed of the postfrontal, postorbital, and squamosal. Posteriorly the boundary is formed by a post-temporal bar, formed by the parietal and squamosal. Below the supratemporal arcade is another large vacuity, the infratemporal or lateral temporal fossa (fig. 52, 21). This is bounded above by the supratemporal arcade, and is separated from the orbit in front by the postorbital bar, formed by the union of outgrowths from the jugals and postorbitals. Behind it is bounded by a continuation of the post-temporal bar formed of the squamosal and quadratojugal, and below by an infratemporal arcade, which is chiefly composed of the quadratojugal and jugal.

Below the post-temporal bar is a third vacuity, the post-temporal fossa (fig. 52, D, 23), bounded above by the post-temporal bar and below by the exoccipital and opisthotic.

Sphenodon and the Crocodilia are the only living reptiles with complete supratemporal and infratemporal arcades, but they are both present in the extinct Pterosauria and some Dinosauria.

Supratemporal fossae, bounded below by supratemporal arcades, occur in all reptiles except some Chelonia, the Ophidia, the Geckonidae among Lacertilia, and the Pariasauria and others among Theromorpha; they are specially large in Nothosaurus among the Sauropterygia, Dicynodon among the Theromorpha, and many Crocodilia and Pterosauria. In some Dinosaurs, such as Ceratosaurus, they are very small, while the infratemporal fossae are correspondingly large.

In Elginia[93] (Theromorpha) and some Chelonia, such as Chelone, there are no fossae on the surface of the skull, a complete false roof being developed; in other Chelonia, such as Trionyx, the true cranium is freely visible, the only part of the false roof developed being the infratemporal arcade.

In many reptiles large pre-orbital vacuities occur; they are specially large in the Pterosauria and in some of the Crocodilia and Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with the orbits.

The premaxillae are usually separate, but sometimes, as in some Ophidia (fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs (Ceratopsia) they are united. In the Dinosaur Hadrosaurus they are exceedingly large and spatulate. In the Rhynchocephalian Hyperodapedon they are drawn out into a strongly curved beak.

As regards the mandible, sometimes, as in most Rhynchocephalia, Ophidia and Pythonomorpha, the rami have only a ligamental union; sometimes, as in Crocodilia, the Rhynchosauridae and the majority of Lacertilia, they are suturally united. In Chelonia (fig. 28, B, 12), and apparently in Pterosauria, the two dentaries are completely fused together. The sutures between the various bones of the lower jaw usually persist, but in Ophidia those between the angular, supra-angular, articular and coronoid are obliterated. There are sometimes large vacuities in the mandible, as in Theromorpha, Crocodilia, and some Dinosauria. In Iguanodon, Polyonax, Hypsilophodon and Hadrosaurus among Dinosaurs the mandible has a predentary or mento-meckelian bone which, in some cases at any rate, was probably sheathed in a horny beak.

The principal part of the auditory ossicular chain is formed by a rod-like columella. The development of the hyoid apparatus varies, and it often happens that the first branchial arch is better developed than is the hyoid arch. In the Crocodilia and Chelonia there is a large basilingual plate or body of the hyoid (fig. 53, 1); but while in the Crocodilia the first branchial forms the only well-developed arch, in the Chelonia the first and second branchials are both strongly developed, and the hyoid is often fairly large.

The Ribs.

Ribs are always present, and may be attached to any of the precaudal vertebrae. In most reptiles the posterior cervical vertebrae bear ribs, while the atlas and axis are ribless; in Crocodiles and Geckos, however, ribs are borne even by the atlas and axis. On the other hand, in the Chelonia none of the cervical vertebrae bear obvious ribs. In the following groups the thoracic ribs have both capitula and tubercula—Theromorpha, Ichthyosauria, Crocodilia, Dinosauria, Pterosauria. In the other groups each rib articulates by a single head, and the position of the facet is subject to a considerable amount of variation, thus in the Squamata it lies on the centrum, and in the Sauropterygia on the neural arch, while in the Chelonia the rib articulates with the contiguous parts of two centra instead of directly with one.

In most reptiles a greater or smaller number of ribs are united ventrally with a sternum; but in snakes a continuous series of similar ribs, all articulating freely with the vertebral column, extends from the third cervical vertebra to the end of the trunk. The number of ribs connected with the sternum varies from three or four in Lizards to eight or nine in Crocodiles. Those which reach the sternum are nearly always divided into vertebral, sternal, and intermediate portions, and as a rule only the vertebral portion is completely ossified. In Crocodiles a number of sternal ribs are connected with a cartilaginous arch, which is attached to the hind end of the sternum, and represents the xiphisternum. The sacral ribs connecting the vertebral column with the ilia are very distinct in Crocodiles; in these animals and Sphenodon the vertebral ribs have backwardly-projecting uncinate processes as in birds.

In the curious arboreal lizard, Draco volans, the posterior ribs are long and straight, and support a parachute-like expansion of the integument used in its long flight-like leaps. In Chelonia the ribs are generally combined with the carapace.

In Ichthyosauria, Sauropterygia, Crocodilia and Sphenodon, abdominal splint ribs occur; and probably all except the first of the paired ossifications forming the plastron of Chelonia are of similar character. Abdominal ribs have quite a different origin from true ribs, for while true ribs are cartilage bones, abdominal ribs have no cartilaginous precursors, but are simply the ossified tendons of the rectus abdominalis muscle.

The Sternum.

A sternum occurs in the following groups of reptiles: Rhynchocephalia, nearly all Lacertilia, Pythonomorpha, Crocodilia, and Pterosauria, and is generally more or less rhomboidal or shield-shaped. In Pterosauria it is keeled and bears some resemblance to that of birds. It may have been replaced by membrane bone.

The sternum is absent in Sauropterygia, Ichthyosauria, Chelonia, Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia; while it is not well known in Theromorpha and Dinosauria. In the Sauropod Brontosaurus, however, two rounded bones occur near the base of the coracoids, and these probably represent ossified patches in a sternum, which was mainly cartilaginous; similar structures occur in Iguanodon.

The sternum frequently remains wholly cartilaginous, especially in Lacertilia; sometimes it becomes calcified, but true ossification does not as a rule take place.

APPENDICULAR SKELETON.

The Pectoral Girdle.

The pectoral girdle is well developed in all groups of reptiles except the Ophidia, occurring even in the limbless Amphisbaenidae. It is very solid in the Theromorpha. As a rule all three cartilage bones, scapula, coracoid, and precoracoid are represented, and frequently also the membrane bones,—clavicles, and interclavicle.

The coracoids are generally flat expanded bones, which sometimes, as in Sauropterygia and Ichthyosauria, meet in a ventral symphysis; sometimes, as in Lacertilia, are united with the sides of the sternum. In Chelonia neither the coracoids nor precoracoids meet one another, but their free ends are connected by fibrocartilaginous bands. In Lacertilia the coracoids are pierced by fenestrae.

The precoracoid is generally represented, but the Theromorpha are the only reptiles in which it is separately ossified; it forms a well-marked process on the coracoid in Lacertilia (fig. 54, 5). It is absent in Ichthyosauria, and Dinosauria, and probably in Sauropterygia. In some Lacertilia and Chelonia the sternal ends of the coracoids are unossified and form epicoracoids; in some Chelonia there are also epiprecoracoids; but neither these nor the epicoracoids overlap their fellows of the opposite side as they do in arciferous Anura (see p. 185). In some Lacertilia with degenerate limbs the pectoral girdle is also much reduced, in Ophisaurus apus the ventral borders of the coracoids are widely separated.

A scapula is always present, and is generally expanded distally, but in the Chelonia the distal end is cylindrical. In the Theromorpha it has an acromial process with which the precoracoid articulates, and it is very large in Dinosauria. In the Chelonia the scapula and precoracoid are ossified continuously. Among the Pterosauria, Pteranodon has an unique pectoral girdle; the scapula and coracoid are ankylosed and the scapula articulates with the neural spines of several ankylosed vertebrae.

Clavicles occur in some Theromorpha such as Pariasaurus, and also in the Ichthyosauria, Sauropterygia, Rhynchocephalia, and most Lacertilia. They are absent in the Pterosauria, the Chamaeleons among Lacertilia, the Ophidia and the Crocodilia. They are wanting too in the Chelonia, unless the first pair of ossifications in the plastron are to be regarded as clavicles. In the Sauropterygia bones regarded as the clavicles and interclavicle are generally well developed. The unpaired ossification in the plastron of Chelonia is an interclavicle, and a representative of the same bone occurs arising from the sternum in Pterosauria. A well developed T-shaped interclavicle is found in Ichthyosauria, Rhynchocephalia, Lacertilia, and some Theromorpha, such as Pariasaurus.

The Limbs.

In most reptiles there are two pairs of pentedactylate limbs provided with claws, but in nearly all Ophidia and some Lacertilia (Amphisbaena, Lialis, Anguis) the limbs have entirely disappeared. In a few Ophidia such as Python traces of the posterior limbs occur, and in Chirotes among the Amphisbaenidae there are minute anterior limbs. The Lacertilians, Chalcides (Seps) and Ophisaurus (Bipes, Pseudopus) have very small posterior limbs.

The limbs are as a rule adapted for walking, but in Ichthyosauria, Sauropterygia, Pythonomorpha and some Chelonia, they have the form of swimming paddles, the relative size of the manus and pes being increased, while that of the proximal and middle portions of the limbs is reduced. This reduction is carried to its furthest extent in the Ichthyosauria in which radius and ulna, tibia and fibula, have the form of short polygonal bones similar to those constituting the manus and pes. In the Pythonomorpha the reduction of the limb bones is not quite so marked, in the Sauropterygia it is less, and still less in the Chelonia. In the earlier Ichthyosauria too, the limb bones are not so short as they are in the later forms. The Ichthyosaurian limb is also remarkable, firstly for the fact that both humerus and femur are terminated by concave articulating surfaces instead of by convex condyles, and secondly for the great multiplication of the phalangeal bones, each digit being sometimes composed of a series of over twenty. Sometimes too the number of series is increased, either by the bifurcation of some of the digits or by the development of marginal bones. In the Sauropterygia the phalanges are likewise increased above the normal but not so much as in Ichthyosauria. The humerus and femur of Sauropterygia are noticeable for the enormous size of the terminal epiphyses which form in each case by far the greater part of the bone.

The Anterior Limb.

The anterior limb is usually approximately equal in length to the posterior, but in many Dinosauria it is considerably the shorter of the two. The humerus is generally without distinct condyles, but they are well developed in the Theromorpha, the Lacertilia and Sphenodon.

In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such as Mesosaurus, the humerus has an ent-epicondylar foramen; in Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar foramen or groove; Sphenodon possesses both ent- and ect-epicondylar foramina. The radius and ulna are always separate. In some Chelonia, such as Chelydra, the carpus has a very simple arrangement, namely, a proximal row of three bones, the radiale, intermedium and ulnare, and a distal row of five carpalia, with one bone, the centrale, between the two rows. Many reptiles have a carpus only slightly different from this. Thus the carpus in Sphenodon differs mainly in having two centralia, that of most Lacertilia, in having the centrale and intermedium fused.

Crocodiles have a much reduced carpus with the radiale and ulnare considerably elongated. The manus in Chamaeleons is curiously modified, having the first three digits arranged in one group and turned inwards, and the fourth and fifth in another group turned outwards; carpalia 3 and 4 are united.

In the Pterosauria the anterior limbs form wings, the phalanges of the fifth digit being very greatly elongated to support the wing membrane. The first digit is vestigial and the second, third, and fourth are clawed.

The Pelvic Girdle.

The pelvic girdle is well developed in all reptiles which have posterior limbs, but is absent or quite vestigial in Ophidia and those Lacertilia which have no posterior limbs. The ilium and ischium agree in their general characters throughout all the various groups of reptiles, but that is not the case with the pubis.

In many reptiles such as Chelonia, Ichthyosauria and Lacertilia the ilia are small, more or less cylindrical bones either directed backwards, or vertically placed as in the Chamaeleons. In the Crocodilia they are larger and more expanded, while in Dinosauria and Pterosauria they are greatly elongated both in front of, and behind, the acetabulum. The ischia are generally strongly developed somewhat square bones meeting in a ventral symphysis. In Dinosauria the ischium (fig. 35, 9) is a much elongated and backwardly-directed bone, bearing a forwardly projecting obturator process. In Pterosauria the ischium is fused with the ilium, and in both pterosaurs and crocodiles the ilium and ischium are the only bones taking part in the formation of the acetabulum. In most Lacertilia there is an unpaired structure, the hypo-ischium or os cloacae projecting back from the symphysis ischii, which is usually separated from the symphysis pubis by a large space, the foramen cordiforme. In some Lacertilia and Chelonia there is a cartilaginous bar dividing the foramen cordiforme into two obturator foramina; in many Chelonia this bar is ossified. Among Ophidia, Python, Tortrix, Typhlops and their allies have a structure representing a vestigial ischio-pubis: but in most Ophidia there is no trace of the pelvis. In some Theromorpha all the bones of the pelvis are completely fused, forming an os innominatum as in mammals; the pubes and ischia are so completely fused that sometimes as in Pariasaurus even the obturator foramina are closed.

Concerning the reptilian pubis there are considerable difficulties. Sometimes there is only a single pubic structure present, sometimes there are two. The reptilian pubis is best understood by comparing the arrangements met with in the various other groups with that in the Orthopod Dinosaurs such as Iguanodon. In Iguanodon the pubis consists of two portions, viz. of a moderately broad pre-pubis directed downwards and forwards, and of a narrow greatly elongated post-pubis directed backwards parallel to the ischium. The pubis is united to both ilium and ischium, the acetabulum has a large unossified space, and neither pre-pubes nor post-pubes meet in ventral symphyses. The arrangement bears a great resemblance to that of Ratite birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria together with Theropod and Sauropod Dinosaurs the pubis corresponds to the pre-pubis of Iguanodon and is a more or less cylindrical bone expanded at both ends, meeting its fellow in a ventral symphysis. In Chelonia and Lacertilia the pubis bears a lateral process which is homologous with the post-pubis of Iguanodon. In Lacertilia and sometimes in Chelonia there is a cartilaginous epipubis attached to the anterior border of the pubic symphysis; this is well developed in the Chamaeleons and Geckos. In Crocodilia there is, forming the anterior and ventral portion of the acetabulum, a patch of cartilage (fig. 49, 3) which is probably the true pubis homologous with that of lizards and with the pre-pubis of Iguanodon. The large bone generally called the pubis in Crocodiles is probably an epipubis.

The Posterior Limb.

The posterior limb is entirely absent in some Lacertilia and in most Ophidia, though traces occur in Python, Tortrix and Typhlops. In the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs form swimming paddles and have been already referred to.

The arrangement of the proximal and middle segments of the limb is fairly constant in all reptiles with limbs adapted for walking, and the tibia and fibula are always separate. The pes is however subject to a considerable amount of variation, especially as regards the tarsus. In some Chelonia the tarsus like the carpus has an extremely simple arrangement, consisting of a proximal row of three bones, the tibiale, intermedium and fibulare, a centrale, and a distal row of five tarsalia. In most living reptiles, however, the tibiale and intermedium are as in mammals united, forming the astragalus. In Crocodiles (fig. 48, B, 15) the centrale is also united with the tibiale while the distal tarsalia are very slightly developed. The calcaneum in Crocodiles is drawn out into a long process forming a heel in a manner almost unique among Sauropsida. In Sphenodon and Lacertilia the tibia and fibula articulate with a single large bone representing the whole proximal row of tarsalia.

The pes is generally pentedactylate, but in some Crocodiles the fifth digit is vestigial (fig. 48, B), and in some Dinosauria (fig. 35) there are only three digits. The North American Dinosaurs present a continuous series ranging from a pentedactylate plantigrade form like Morosaurus, to such a form as Hallopus with a highly digitigrade and specialised pes reduced to three functional digits, and a vestigial fifth metatarsal. The second, third and fourth metatarsals in this form are nearly two-thirds as long as the femur, and the calcaneum is drawn out into a heel much as it is in most mammals.

In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to lie between the proximal and distal row of tarsals as in birds.