The abdominal splint ribs.

Lying superficially to the recti muscles of the ventral body-wall, behind the sternal ribs, are seven or eight series of slender curved bones, the abdominal ribs (fig. 46, 4). Each series consists of four or more bones, arranged in a V-like form with the angle of the V directed forwards. They show a considerable amount of variability in number and character. They are really membrane bones, and are in no way homologous with true ribs, but correspond rather with the more posterior of the bones constituting the plastron of Chelonia.

2. The Appendicular Skeleton.

This includes the skeleton of the two pairs of limbs and their respective girdles.

The Pectoral girdle.

The pectoral girdle of the Crocodile is less complete than is that of most reptiles. It consists of a dorsal bone, the scapula, and a ventral bone, the coracoid, with a median unpaired element, the interclavicle; but there is no separate representative either of the clavicle or precoracoid.

The scapula (fig. 47, 1) is a large bone, flattened and expanded above where it is terminated by an unossified margin, the suprascapula, and thickened below where it meets the coracoid. The scapula forms about half the glenoid cavity (fig. 47, 4) for articulation with the humerus, and has the lower part of its anterior border drawn out into a roughened ridge.

The coracoid (fig. 47, 2) is a flattened bone, much expanded at either end; it bears on its upper posterior border a flattened surface which forms half the glenoid cavity, and is firmly united to the scapula at its dorsal end. Its ventral end meets the sternum.

The interclavicle (figs. 46, 1, and 47, 3) is a long narrow blade-shaped bone lying along the ventral side of the sternum; about a third of its length projects beyond the sternum in front.

The Anterior limb.

This is as usual divisible into three portions, the upper arm, fore-arm and manus.

The upper arm or brachium contains one bone, the humerus.

The humerus (fig. 48, A, 1) is a fairly long stout bone, considerably expanded at either end. The proximal end or head is evenly rounded and is formed by an epiphysis ossifying from a centre different from that forming the shaft. It articulates with the glenoid cavity. The shaft bears on the flexor surface, at some little distance behind the head, a prominent rounded protuberance, the deltoid ridge. The distal end or trochlea is also formed by an epiphysis and is partially divided by a groove into two convex surfaces; it articulates with the two bones of the fore-arm, the radius and ulna.

The radius and ulna are nearly equal in size and each consists of a long shaft terminated at either end by an epiphysis.

The radius (fig. 48, A, 2) or pre-axial bone is slightly the smaller of the two. It has a straight cylindrical shaft and is slightly and nearly evenly expanded at either end. The proximal end which articulates with the humerus is flat or slightly concave, the distal end which articulates with the carpus is slightly convex.

The ulna (fig. 48, A, 3) or postaxial bone is a curved bone rather larger than the radius. Its proximal end is large and convex, but is not drawn out into an olecranon process.

The Manus consists of the carpus or wrist, and the hand.

The Carpus. This differs considerably from the more primitive type met with in the Turtle. It consists of six elements arranged in a proximal row of three and a distal row of two, with one intervening. The bones of the proximal row are the radiale, the ulnare, and the pisiform. The radiale (fig. 48, A, 4) is the largest bone of the carpus: it is a somewhat hour-glass shaped bone, with its ends formed by flattened epiphyses. It articulates by its proximal end with the whole of the radius, and partly also with the ulna, and by its distal end with the centrale.

The ulnare (fig. 48, A, 5) is a smaller bone, also somewhat hour-glass shaped; it articulates proximally with the pisiform and radiale, not quite reaching the ulna. The third bone of the proximal row is the pisiform (fig. 48, A, 6), an irregular bone, articulating with the ulna, radiale, and fifth metacarpal. The centrale is a flattened cartilaginous element applied to the distal surface of the radiale.

The distal row of carpals consists of two small structures. The first of these forms a small cartilaginous patch, which is wedged in between the first and second metacarpals, the centrale and the bone representing carpalia 3, 4 and 5; this cartilaginous patch represents carpalia 1 and 2 (fig. 48, A, 7). The bone representing carpalia 3, 4 and 5 is a good deal larger, rounded, and well-ossified; it articulates with the ulnare, the pisiform, and the third, fourth, and fifth metacarpals.

The hand. Each of the five digits consists of an elongated metacarpal, terminated at each end by an epiphysis, and of a varying number of phalanges. The terminal phalanx of each digit has an epiphysis only at its proximal end, the others have them at both ends.

The first digit, or pollex, is the stoutest, and has two phalanges, the second has three, the third four, the fourth three, and the fifth two. The terminal phalanx of each of the first three digits is pointed and sheathed in a horny claw; and is also marked by a pair of prominent lateral grooves.

The Pelvic Girdle.

The pelvic girdle of the Crocodile consists of four parts, a dorsal element, the ilium, an anterior ventral element, the pubis, a posterior ventral element, the ischium, and an accessory anterior ventral element, the epipubis. All except the epipubis take part in the formation of the acetabulum, which is perforated by a prominent hole.

The ilium (fig. 49, 1) is a thick strong bone, firmly united on its inner side with the two sacral ribs. Its dorsal border is rounded, its ventral border bears posteriorly two irregular surfaces, completed by epiphyses, which are united respectively with the ischium and pubis.

The ischium (fig. 49, 2)—the largest bone of the pelvis, is somewhat contracted in the middle and expanded at either end. Its proximal end, which is formed by an epiphysis, bears two surfaces, one of which is united to the ilium, while the other forms part of the acetabulum. The anterior border is also drawn out dorsally into a strong process, which is terminated by a convex epiphysis, and is united to the pubis. The ventral end of the ischium forms a flattened blade, meeting its fellow in a median symphysis.

The pubis (fig. 49, 3) is much smaller than either the ilium or ischium; it forms a small patch of unossified cartilage, interposed between the anterior parts of the ilium and ischium.

The epipubis (fig. 49, 4) is a large bone with a thickened proximal end, which is loosely articulated to the ischium, and a flattened expanded distal end, which is united with its fellow, and with the last pair of abdominal ribs by a large plate of cartilage. This bone is generally described as the pubis.

The Posterior limb.

This is as usual divisible into three portions, the thigh, the crus or shin, and the pes.

The thigh is formed by the femur (fig. 48, B, 12), a moderately long stout bone, not unlike the humerus; it articulates with the acetabulum by a fairly prominent rounded head. The distal end articulating with the tibia and fibula is also expanded, and is partially divided into equal parts by anterior and posterior grooves. The flexor surface bears a fairly prominent trochanteric ridge. Each end of the femur is formed by an epiphysis.

The crus or shin includes two bones, the tibia and fibula. Both are well developed, but the tibia is considerably the larger of the two.

The tibia (fig. 48, B, 13) is a strong bone with a flattened expanded proximal end articulating with almost the whole of the end of the femur, and a similarly expanded distal end articulating with a bone representing the fused astragalus and centrale.

The fibula (fig. 48, B, 14) is flattened proximally, and articulates with only quite a small part of the femur, while distally it is more expanded, and articulates with the fibulare (calcaneum) and with a facet on the side of the fused astragalus and centrale.

The Pes consists of the tarsus or ankle, and the foot.

The Tarsus. This, like the carpus, is much reduced and modified from the primitive condition. It consists of only four bones, arranged in two rows of two each. The two bones of the proximal row are much larger than are those of the distal row. The pre-axial of them (fig. 48, B, 15) representing the fused astragalus (tibiale and intermedium) and centrale, articulates proximally with the tibia and fibula, and distally with the first metatarsal, and a small bone representing the first three tarsalia. The postaxial bone, the calcaneum (fibulare) (fig. 48, B, 16), is drawn out into a prominent posterior process forming a heel such as is almost unknown elsewhere except in mammals. It articulates with the fibula, the tibiale-centrale, and distally with a bone representing the fourth and fifth tarsalia, and with the fifth metatarsal.

The two bones forming the distal row of tarsals are both small and rounded; one represents the first three tarsalia fused together, the other tarsalia 4 and 5.

The Foot. The foot has five digits, but the fifth is much reduced, consisting only of a short metatarsal. The first four metatarsals are all long bones, slightly expanded at each end, and terminated by small epiphyses. The first digit has two phalanges, the second three, the third four, and the fourth five. The terminal or ungual phalanx in each instance is grooved and pointed, and in the case of the first three digits bears a horny claw. The ungual phalanx progressively decreases in size from the first to the fourth. The fifth digit consists only of a small, somewhat square metatarsal (fig. 48, B, 21), attached to the bone representing the fused fourth and fifth tarsalia.

CHAPTER XVI.
GENERAL ACCOUNT OF THE SKELETON IN REPTILES.

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.