This great group includes the Reptiles and Birds and forms the second of the three into which the Gnathostomata may be divided. There is nearly always a strongly-developed epiblastic exoskeleton which has the form of scales or feathers, and in some cases a dermal exoskeleton is also well developed. In living forms the notochord never persists, being replaced by vertebrae, but in some extinct forms the centra are notochordal. The vertebral centra are ossified, and only in exceptionally rare cases have terminal epiphyses. The skull is well ossified and has membrane bones incorporated in its walls.

The occipital segment is completely ossified, and an interorbital septum or bony partition separating the two orbits is usually developed to a greater or less extent. The skull generally articulates with the vertebral column by a single occipital condyle into the composition of which the exoccipitals and basi-occipital enter in varying proportions. The pro-otic ossifies, and either remains distinct from the epi-otic[65] and opisthotic throughout life, or unites with them only after they have fused with the adjacent bones. The hyoid and branchial arches are much reduced; and the representative of the hyomandibular is connected with the auditory apparatus, forming the auditory ossicles[66]. Each ramus of the mandible always consists of a cartilage bone, the articular, and several membrane bones. The mandible articulates with the cranium by means of a quadrate. The ribs in Birds and some Reptiles bear uncinate processes, i.e. small, flat, bony or cartilaginous plates projecting backwards from their posterior borders. The sternum is not transversely segmented as in mammals, and there are commonly distinct cervical ribs. The ankle joint is intertarsal, or situated between the proximal and distal row of tarsal bones, not cruro-tarsal as in Mammalia.

Class I. Reptilia[67].

The axial skeleton is generally long, and that of the limbs frequently comparatively short, or sometimes absent.

The exoskeleton generally has the form of epidermal scales, which are often combined with underlying bony dermal plates or scutes and may sometimes form a continuous armour. Neither feathers nor true hairs are ever present. The vertebral column is generally divisible into the five usual regions. The centra of the vertebrae vary enormously, and may be amphicoelous, procoelous, opisthocoelous or flat, but they never have saddle-shaped articulating surfaces. The quadrate is always large, and is sometimes fixed, sometimes movable. A transpalatine bone uniting the pterygoid and maxillae is generally present.

Free ribs are often borne along almost the whole length of the trunk and tail, and often occur attached to the cervical vertebrae. The sacrum is generally composed of two vertebrae which are united with the ilia by means of expanded ribs. The sternum is rhomboidal, and may either be cartilaginous or formed of cartilage bone, but never of membrane bone; it differs from that of birds also in the fact that it does not ossify from two or more centres. An interclavicle is generally present. There are always more than three digits in the manus, and never less than three in the pes. In all living reptiles the ilia are prolonged further behind the acetabula than in front of them, and the bones of the pelvis remain as a rule, distinct from one another throughout life.

The pubes (pre-pubes) and ischia both commonly meet in ventral symphysis, and the acetabula are wholly or almost wholly ossified. The metatarsals are not ankylosed together.

Order 1. Theromorpha[68].

This order includes a number of mainly terrestrial, extinct reptiles, which differ much from one another, and show remarkable points of affinity on the one hand with the Labyrinthodont Amphibia, and on the other with the Mammalia. The vertebrae are nearly always amphicoelous and sometimes have notochordal centra. The skull is short and has the quadrate immovably fixed. There is an interparietal foramen, and generally large supratemporal fossae bounded by supratemporal arcades, but with no infratemporal[69] arcades; Elginia however has the whole of the temporal region completely roofed over.

The teeth are placed in distinct sockets and are very variable in form, the dentition sometimes resembling the heterodont dentition of mammals. The humerus has distinct condyles and an ent-epicondylar foramen[70] as in many mammals.

The pubis is fused with the ischium, and both pectoral and pelvic girdles are remarkably solid. The obturator foramen is remarkably small or even absent. The anterior ribs have two articulating surfaces, and each articulates by its tuberculum with the transverse process, and by its capitulum with the centrum as in mammals.

These reptiles occur chiefly in deposits of Triassic and Permian age. Some of the best known genera are Dicynodon, Udenodon, Placodus, Pariasaurus and Galesaurus. They will be noticed in the general account of the skeleton in reptiles.

Order 2. Sauropterygia.

This order includes a number of extinct marine reptiles, devoid of an exoskeleton. The tail is short, the trunk long, and the neck in the most typical forms extremely long. The vertebrae have slightly biconcave, or nearly flat centra. The skull is relatively small and has large supratemporal fossae. The teeth are placed in distinct sockets, and are generally confined to the margins of the jaws; they are sharp and curved and are coated with grooved enamel. The premaxillae are large, and there is an interparietal foramen. The quadrate is firmly united to the cranium. The anterior nares are separate and are placed somewhat close to the orbits. There is no ossified sclerotic ring. The palatines and pterygoids meet the vomers, and more or less completely close the palate, and in some forms, e.g. Plesiosaurus, there is a distinct parasphenoid. Thoracic ribs are strongly developed and each articulates with its vertebra by a single head. The cervical vertebrae have well-marked ribs, which articulate only with the centra, in this respect differing from those of Crocodiles. The caudal vertebrae bear both ribs and chevron bones, and abdominal splint-ribs are largely developed.

In the shoulder-girdle the coracoids are large and meet in a ventral symphysis; precoracoids and a sternum are apparently absent, but parts generally regarded[71] as the clavicles and interclavicle are well developed. In the pelvis, the pubes and ischia meet in a long symphysis. The limbs are pentedactylate, and in the best known forms, the Plesiosauridae, form swimming paddles.

The Sauropterygia occur in beds of Secondary age, and some of the best known genera are Plesiosaurus, Pliosaurus and Nothosaurus.

Order 3. Chelonia.

In the Tortoises and Turtles the body is enclosed in a bony box, formed of the dorsal carapace, and a flat ventral buckler, the plastron. Except in Dermochelys the carapace is partly formed from the vertebral column and ribs, partly from dermal bones. Both carapace and plastron are, except in Dermochelys, Trionyx and their allies, covered with an epidermal exoskeleton of horny plates, which are regularly arranged, though their outlines do not coincide with those of the underlying bones. The thoracic vertebrae have no transverse processes, and are quite immovably fixed, but the cervical and caudal vertebrae are very freely movable. There are no lumbar vertebrae. The skull is extremely solid, and frequently has a very complete false roof. Teeth have been detected in embryos of Trionyx but with this exception the jaws are toothless, and are encased in horny beaks. The quadrate is firmly fixed. The facial part of the skull is very short, and the alisphenoidal and orbitosphenoidal regions are unossified. In living forms there are no separate nasal bones, while large prefrontals and postfrontals are developed. There is a comparatively complete bony palate chiefly formed of the palatines and pterygoids. The anterior nares are united and placed at the anterior end of the skull, and the premaxillae are very small. There is no transpalatine bone and the vomer is unpaired. The dentaries are generally fused together.

There are ten pairs of ribs, and each rib has only a single head and is partially attached to two vertebrae; there are no cervical or sternal ribs. There is no true sternum.

The three anterior elements of the plastron are respectively homologous with the interclavicle and two clavicles of other reptiles, while the remaining elements of the plastron are probably homologous with the abdominal ribs of Crocodiles. The pectoral girdle lies within the ribs, and the precoracoids and coracoids do not meet in ventral symphyses. The scapula and precoracoid are ossified continuously. The pubis probably corresponds with the pre-pubis of Dinosaurs. There are four limbs each with five digits.

The order includes three suborders:—

Suborder (1). Trionychia.

The carapace and plastron have a rough granular surface covered with skin and without any horny shields.

The plastron is imperfectly ossified, and marginal bones may be absent, or if present are confined to the posterior portion of the carapace. The pelvis is not united to the plastron. The cranium has not a complete false roof and the head can be drawn back under the carapace.

The first three digits of both manus and pes bear claws, and the fourth digit in each case has more than three phalanges. The most important genus is Trionyx.

Suborder (2). Cryptodira.

The carapace and plastron vary in the extent to which they are ossified, and except in Dermochelys[72] and its allies are covered by horny plates. Marginal bones are always present. The head can generally be drawn back under the carapace. The pelvis is not firmly united to the plastron. The cranium often has a complete false roof, and in the mandibular articulation the cup is borne by the cranium, and the knob by the mandible. Among the more important genera are Dermochelys, Chelone, and Testudo.

Suborder (3). Pleurodira.

The carapace and plastron are strongly ossified, and firmly united to the pelvis. The head and neck can be folded laterally under the carapace, but cannot be drawn back under it. The cranium has a more or less complete false roof, and in the mandibular articulation the knob is borne by the cranium, and the cup by the mandible. Chelys is a well-known genus.

Order 4. Ichthyosauria[73].

The order includes a number of large extinct marine reptiles whose general shape is similar to that of the Cetacea. The skull is enormously large, and the neck short. The tail is very long, and is terminated by a large vertically-placed bilobed fin, the vertebral column running along the lower lobe. The very numerous vertebrae are short and deeply biconcave. The vertebral column can be divided into caudal and precaudal regions only, as the ribs which begin at the anterior part of the neck are continued to the posterior end of the trunk without being connected with any sternum or sacrum. The precaudal vertebrae bear two surfaces for the articulation of the ribs, while in the caudal vertebrae the two surfaces have coalesced. The caudal region is also distinguished by its chevron bones. The vertebrae have no transverse processes, and the neural arches are not firmly united to the centra, and have only traces of zygapophyses. The atlas and axis are similar to the other vertebrae, but there is a wedge-shaped intercentrum between the atlas and the skull, and another between the atlas and the axis. The skull is greatly elongated (fig. 32) and pointed, mainly owing to the length of the premaxillae. The orbits are enormous, and there is a ring of bones in the sclerotic (fig. 32, 15). The anterior nares are very small; and are placed far back just in front of the orbits. There is an interparietal foramen, and the supratemporal fossae (fig. 32, 9) are very large, while there are no infratemporal fossae. An epipterygoid occurs. The quadrate is firmly fixed to the cranium, and there is a large parasphenoid. There are large prefrontals, but the frontals are very small. The very numerous teeth are large and conical, and are placed in continuous grooves without being ankylosed to the bone. They are confined to the jaw-bones.

The ribs are long, and the anterior ones have capitula and tubercula. There is no sternum, but the ventral body wall is strengthened by a complex system of abdominal splint ribs.

The pectoral girdle is strongly developed, the scapulae are narrow, the coracoids broad, and meet ventrally without overlapping. There are probably no precoracoids, but clavicles and a T-shaped interclavicle are well developed.

The limbs are very short, and completely modified into swimming paddles. The humerus and femur are both short, while the radius and ulna, tibia and fibula are generally still further reduced to the form of short polygonal bones.

The digits are formed of longitudinal series of very numerous small bones. The number of digits is five, but there sometimes appear to be more owing to the bifurcation of certain of them, or to the addition of marginal bones, either to the radial or ulnar side of the limb. The humerus has no foramen, and both humerus and femur are unique in that they are distally terminated by concave surfaces instead of by convex condyles. The pelvic limb is much smaller than the pectoral. The pelvis has no bony connection with the vertebral column, and all the component bones are small and rod-like.

The Ichthyosauria are confined to beds of Secondary age and by far the best known genus is Ichthyosaurus.

Order 5. Rhynchocephalia.

This order includes the living Sphenodon (Hatteria) and various extinct forms. The general shape of these animals is lizard-like and the tail is long.

The vertebrae are amphicoelous or sometimes nearly flat, and the notochord sometimes persists to some extent. Proterosaurus differs from the other members of the order in having opisthocoelous cervical vertebrae.

The sacrum is composed of two vertebrae. Ossified inter centra (interdorsalia) generally occur in the cervical and caudal regions, and sometimes throughout the whole vertebral column. In the skull the quadrate is immovably fixed and united to the pterygoid. The palate is well ossified, while the premaxillae which are often beak-like are never ankylosed together. The jaws may be toothless or may be provided with teeth which are usually acrodont (see p. 199). The palatines frequently bear teeth, and in Proterosaurus teeth occur also on the pterygoids and vomers. The rami of the mandible are united by ligament at the symphysis except in the Rhynchosauridae, in which the union is bony. Superior and inferior temporal arcades occur.

The ribs have capitula and tubercula, and often uncinate processes (see p. 190) as in birds. A pectoral girdle and sternum, with clavicles and a T-shaped interclavicle are developed, and abdominal ribs are always found. The precoracoid is however absent. The limbs are pentedactylate.

Sphenodon[75] (Hatteria) now living in some of the islands of the New Zealand group, is certainly the most generalised of all living reptiles. Though lizard-like in form it differs from all living lizards in the possession of two temporal arcades, abdominal ribs and a fixed quadrate; and is often considered to be nearly allied in many respects to the type of reptile from which all the others took their origin.

Among the better known extinct forms are Proterosaurus of Permian and Hyperodapedon of Triassic age.

Order 6. Squamata.

This order includes the extinct Mosasaurians, and the lizards and snakes which form the vast majority of living reptiles. The trunk may be moderately elongated and provided with four short limbs as in lizards, or it may be limbless, extremely elongated, and passing imperceptibly into the tail. The surface is generally completely covered with overlapping horny epidermal scales, below which bony dermal scutes may be developed.

The vertebrae are procoelous, rarely amphicoelous. There are no inter centra, and the neural arches are firmly united to the centra. Additional articulating surfaces, the zygosphenes and zygantra, are often developed[76]. The sacrum is formed of two or rarely three vertebrae, or may be wanting as in Ophidia. In the skull an infratemporal arcade forming the lower boundary of the infratemporal fossa is absent, and the quadrate, except in the Chamaeleons, is movably articulated to the squamosal. The palatal vacuities are large and the nares are separate. There is often a distinct parasphenoid. The teeth are either acrodont (i.e. ankylosed to the summit of the jaw), or pleurodont, i.e. ankylosed to the inner side of the jaw. The thoracic ribs each have a single head which articulates with the centrum of the vertebra; while uncinate processes and abdominal ribs never occur.

A pectoral girdle and sternum may be present, or may be completely absent as in snakes. Except in snakes there are generally four pentedactylate limbs which may either form paddles or be adapted for walking.

Suborder (1). Lacertilia[77].

The body is elongated, and as a rule four short pentedactylate limbs are present, but sometimes limbs are vestigial or absent. The exoskeleton generally has the form of horny plates, spines, or scales; while sometimes as in the Chamaeleons and Amphisbaenians it is absent. In other forms such as Tiliqua and Scincus, the body has a complete armour of bony scutes, whose shape corresponds with that of the overlying horny scales.

The vertebrae are procoelous, rarely as in the Geckos amphicoelous; they are usually without zygosphenes and zygantra, but these structures occur in the Iguanidae. The sacral vertebrae of living forms are not ankylosed together, and the caudal vertebrae usually have well-developed chevron bones.

In the skull[78] the orbits are separated from one another, only by an imperfectly developed interorbital septum, the cranial cavity not extending forwards between them, while the alisphenoidal region is unossified. The premaxillae may be paired or united (Amphisbaenidae), and there is usually an interparietal foramen. There may be a complete supratemporal[79] arcade bounding the lower margin of the supratemporal fossa, or the supratemporal fossa may be open below. The quadratojugal is not ossified, and the quadrate articulates with the exoccipital. There is no infratemporal arcade. There is commonly a rod-like epipterygoid[80] (fig. 33, 14) connecting the pterygoid and parietal.

Teeth are always present, and may be confined to the jaws or may be developed also on the pterygoids and rarely on the palatines; they are either acrodont or pleurodont. The rami of the mandible are suturally united.

A pectoral girdle is always present, and generally also a sternum. Clavicles and a T-shaped interclavicle are commonly present, but are absent in the Chamaeleons.

There is no separate precoracoid but a precoracoidal process (fig. 34, 7) of the coracoid is generally prominent.

Sternal ribs are present in chamaeleons and scinks. The limbs are in the great majority of cases pentedactylate and the digits are clawed. The phalanges articulate by means of condyles. Sometimes one or both pairs of limbs are absent. When the posterior limbs are absent the pelvis is also wanting, though the loss of the anterior limbs does not lead to a corresponding loss of the pectoral girdle.

The pubis corresponds to the pre-pubis of Dinosaurs, and both pubes and ischia meet in ventral symphyses.

The suborder includes the Lizards, Chamaeleons and Amphisbaenians.

Suborder (2). Ophidia[81].

The Ophidia or snakes are characterised by their greatly elongated body and want of limbs. The body is covered with overlapping horny scales and bony dermal scutes are never present. The vertebrae are procoelous, and are distinguishable into two groups only, precaudal or rib-bearing, and caudal or ribless. The atlas vertebra is also ribless. The neural arches are always provided with zygosphenes and zygantra. Many of the vertebrae have strong hypapophyses, and the caudal vertebrae are without chevron bones.

In the skull the cranial cavity extends forwards between the orbits, and is closed in front by downgrowths from the frontals and parietals which meet the well-ossified alisphenoids and orbitosphenoids[82]. The cranium is strongly ossified, and there are no parotic processes or interparietal foramen. There are no temporal arcades and no epipterygoid. The premaxillae if present are very small (fig. 51, 1) and usually toothless. The quadrates articulate with the squamosals, and do not as in Lacertilia meet the exoccipitals. The palatines do not unite directly with the vomers or with the base of the cranium, and the whole palato-maxillary apparatus is more loosely connected with the cranium than it is in Lacertilia. The pterygoids, and in most cases also the palatines, bear teeth. The dentition is acrodont, and the rami of the mandible are united only by an elastic ligament—an important point serving to distinguish the Ophidia from the Lacertilia. There is an imperfectly developed interorbital septum, the ventral part of which is formed by the parasphenoid. The postfrontal is generally well developed, while the jugals and quadratojugals are absent. There are never any traces of the anterior limbs or pectoral girdle, but occasionally there are vestiges of a pelvis and posterior limbs.

Suborder (3). Pythonomorpha[83].

This suborder includes Mosasaurus and its allies, a group of enormous extinct marine reptiles found in beds of Cretaceous age.

The skin is in most forms at any rate unprovided with dermal scutes. The vertebrae may be with or without zygosphenes and zygantra. The skull resembles that of lizards, having an interparietal foramen, and a cranial cavity open in front. The squamosal takes part in the formation of the cranial wall, and the quadrate articulates with the squamosal, not as in Lacertilia with the exoccipital. There are large supratemporal fossae, bounded below by supratemporal arcades. The teeth are large and acrodont, and occur on the pterygoids as well as on the jaws. The two rami of the mandible are united by ligament only. Pectoral and pelvic girdles are present, but clavicles are wanting, and the pelvis is not as a rule united to any sacrum.

The limbs are pentedactylate, and are adapted for swimming, while all the limb bones except the phalanges are relatively very short. The number of phalanges is not increased beyond the normal, and they articulate with one another by flat surfaces. The terminal phalanges are without claws.

Order 7. Dinosauria[84].

The extinct reptiles comprising this order were all terrestrial, and include the largest terrestrial animals known. They vary greatly in size and in the structure of the limbs, some approach close to the type of structure met with in birds, others are allied to crocodiles.

Passing to the more detailed characters:—there is sometimes a well-developed exoskeleton having the form of bony plates or spines. The vertebrae may be solid or their centra may be hollowed internally; their surfaces may be flat, biconcave or opisthocoelous. The sacrum is composed of from two to six vertebrae.

As regards the skull, the quadrate is large and fixed, and supratemporal and infratemporal fossae bounded by bone occur. The teeth are more or less laterally compressed, and often have serrated edges; they may be placed in distinct sockets or in a continuous groove. The ribs have capitula and tubercula, and sternal ribs often occur. The scapula is very large, the coracoid small, and there is no precoracoid, or T-shaped interclavicle. Clavicles are only known in a few cases. In the pelvis the ilium is elongated both in front of, and behind, the acetabulum, sometimes the pre-pubis, sometimes the post-pubis is the better developed. The anterior limbs are shorter than the posterior, and the long bones are sometimes solid, sometimes hollow.

There are three well-marked suborders of the Dinosauria.

Suborder (1). Sauropoda[85].

The reptiles belonging to this group were probably quadrupedal and herbivorous.

They have the cervical and anterior trunk vertebrae opisthocoelous, while the posterior vertebrae are biconcave; all the presacral, and sometimes the sacral vertebrae are hollowed internally. The teeth are spatulate and without serrated edges, they are always planted in distinct sockets, and some of them are borne by the premaxillae.

The nares have the form of long slits and there are large pre-orbital vacuities.

The limb bones are solid, and the anterior limbs are not much shorter than the posterior ones. All the limbs are plantigrade and pentedactylate, and the digits of the pes are clawed. There is a large pre-pubis directed downwards and forwards, meeting its fellow in a ventral symphysis, but there is no post-pubis.

The Sauropoda are found in the secondary rocks of Europe and N. America and include the largest land animals that are known to have existed. Many of the best known forms such as Brontosaurus and Morosaurus are North American.

Suborder (2). Theropoda.

The members of this suborder were all carnivorous, and from the small comparative size of the anterior limbs many of them were probably bipedal.

The vertebrae are opisthocoelous or amphicoelous, their neural arches are provided with zygosphenes and zygantra, and their centra are frequently hollowed internally; the limb bones are also hollow, and in fact the whole skeleton is extremely light. The tail is of great length. The teeth are pointed and recurved, and have one or both borders serrated; they are always planted in distinct sockets, and some of them are borne by the premaxillae. There are large pre-orbital vacuities. The digits of both manus and pes are terminated by pointed ungual phalanges which must have borne claws. In the pelvis the pre-pubes and ischia are slender bones, the former meeting in a ventral symphysis. The ilia are very deep vertically and there are no post-pubes. The astragalus is closely applied to the tibia, in front of which it sends an ascending process, sometimes the two bones appear to have been ankylosed together, as in birds. The metatarsals are elongated and the feet digitigrade.

The Theropoda vary greatly in size, one of the best known genera Compsognathus was about as large as a cat, another, Megalosaurus, perhaps as large as an elephant. Ceratosaurus is the name of a well-known North American form regarded by many authorities as identical with Megalosaurus.

Suborder (3). Orthopoda.

This suborder includes the most specialised of the Dinosaurs, certain of which resemble the Theropoda in being bipedal. In some of them such as Stegosaurus the exoskeleton is strongly developed, in others such as Iguanodon it is absent.

The vertebrae are solid and may be opisthocoelous, biconcave, or flat. The teeth are compressed and serrated, often irregularly, and are frequently not set in distinct sockets. The anterior part of the premaxillae is without teeth, and a toothless predentary or mento-meckelian bone is present. The pre-orbital vacuities are small or absent, and the nares are large and placed far forwards.

The most characteristic features of the group are found in the pelvis which, except in the Ceratopsia, bears a striking resemblance to that of birds. The ischium and post-pubis are long slender bones directed backwards parallel to one another, and the pre-pubis is also well developed. The ischium has an obturator process. The limb bones are sometimes hollow, sometimes solid. The anterior limbs are much shorter than the posterior, pointing to a bipedal method of progression. The pes is digitigrade or plantigrade, and has three, rarely four, digits.

The suborder Orthopoda may be further subdivided into three sections:—

A. Stegosauria.

A dermal exoskeleton is strongly developed. The vertebral centra are flat or biconcave, and neither they nor the limb bones are hollowed out by internal cavities. The limbs are plantigrade, the anterior ones short, the posterior ones very large and strong. The post-pubis is well developed;

e.g. Stegosaurus from the Upper Jurassic of Colorado.

B. Ceratopsia.

There is sometimes a well-developed dermal exoskeleton formed of small granules and plates of bone. The bones are solid, and the vertebral centra flat. The cranium bears a pair of enormous pointed frontal horns, and the parietal is greatly expanded and elevated behind, forming with the squamosals a shield which overhangs the anterior cervical vertebrae. The premaxillae are united, and in front of them is a pointed beak-like bone which bites upon a toothless predentary ossification of the mandible. The teeth have two roots. The anterior limbs are but little shorter than the posterior ones. There is no post-pubis;

e.g. Polyonax from the uppermost Cretaceous of Montana.

C. Ornithopoda[86].

There is no dermal exoskeleton. The cervical vertebrae are opisthocoelous, and so are sometimes the thoracic. The limb bones are hollow and the anterior limbs are much shorter than the posterior ones. The feet are digitigrade and provided with long pointed claws. The post-pubis is long and slender and directed back parallel to the ischium;

e.g. Iguanodon from the European Cretaceous.

Order 8. Crocodilia[87].

This order includes the Crocodiles, Alligators and Garials and various extinct forms, some of which are closely allied to the early Dinosaurs.

There is always a more or less complete exoskeleton formed of bony scutes overlain by epidermal scales; these bony scutes are specially well developed on the dorsal surface but may occur also on the ventral. The vertebral column is divisible into the five regions commonly distinguishable. In all living forms the vertebrae, with the exception of the atlas and axis, the two sacrals, and first caudal, are procoelous, but in many extinct forms they are amphicoelous. The atlas (fig. 71) is remarkable, consisting of four pieces, and the first caudal is biconvex.

The teeth are, in the adult, planted in separate deep sockets. The skull is very dense and solid, and all the component bones including the quadrate are firmly united. The dorsal surface of the skull is generally characteristically sculptured. There is an interorbital septum, and the orbitosphenoidal and presphenoidal regions are imperfectly ossified. Supratemporal, infratemporal, and post-temporal fossae occur, but no interparietal foramen. In living genera there is a long secondary palate formed by the meeting in the middle line of the palatines, pterygoids and maxillae (fig. 43, A).

Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate with rather prominent surfaces borne on the neural arches and centra respectively. The thoracic ribs articulate with the long transverse processes, and sternal ribs and abdominal splint ribs (fig. 46, 4) occur. The sternum is cartilaginous, and both it and the shoulder-girdle are very simple. The precoracoid is represented by merely a small process on the coracoid, while the clavicles are absent, except in the Parasuchia. In the pelvis (fig. 49) there is a large ilium, and an ischium meeting its fellow in a ventral symphysis; these two bones form almost the whole of the acetabulum. In front of the acetabulum, in the Eusuchia, projects a bone which is generally called the pubis, but is in reality rather an epipubis (fig. 49, 4), the true pubis being probably represented by a fourth element which remains cartilaginous for some time, and later on ossifies and attaches itself to the ischium. The limbs are small in proportion to the size of the body, and are adapted for swimming or for shuffling along the ground; they are plantigrade and the bones are all solid. In living forms the anterior limbs have five digits and the posterior four, the fifth being represented only by a short metatarsal. The first three digits in each case are clawed. The calcaneum has a large backwardly-projecting process.

The order Crocodilia may be subdivided into two suborders.

Suborder (1). Parasuchia.

The vertebral centra are flat or biconcave. The premaxillae are very large, and the nares are separated, and placed far back. The posterior narial openings lie comparatively far forward between the anterior extremities of the palatines.

The palatines and pterygoids do not form a secondary palate. The supratemporal fossae are small, and open posteriorly, the lateral temporal fossae are very large. The parietals and frontals are paired. Clavicles are present. The best known and most important genus of these extinct crocodiles is Belodon.

Suborder (2). Eusuchia.

The vertebrae are either biconcave or procoelous. The premaxillae are small, and the anterior nares are united and placed far forwards. The posterior nares lie far back, the palatines and in living genera the pterygoids, meeting in the middle line, and giving rise to a closed palate. The supratemporal fossae are surrounded by bone on all sides, and the parietals, and often also the frontals are united. There are no clavicles. The suborder includes the genera Crocodilus, Alligator, Garialis and others living and extinct.

Order 9. Pterosauria[88].

These animals, called also the pterodactyles or Ornithosauria, are a group of extinct reptiles, whose structure has been greatly modified from the ordinary reptilian type for the purpose of flight.

The skin was naked and they vary greatly in size and in the length of the tail. The vertebrae and limb bones are pneumatic just as in birds. The presacral vertebrae are procoelous and have their neural arches firmly united to the centra. The neck is long, the caudal vertebrae are amphicoelous, and from three to five vertebrae are fused together in the sacral region. The skull is large and somewhat bird-like, the facial portion being much drawn out anteriorly, and the sutures being obliterated. It resembles that of other reptiles in having large supratemporal fossae; large pre-orbital vacuities also occur. The jaws may be toothed or toothless, and the teeth, when present, are imbedded in separate sockets. The premaxillae are large, and the quadrate is firmly attached to the skull. The rami of the mandible are united at the symphysis, and there is an ossified ring in the sclerotic. The occurrence of a postfrontal and its union with the jugal behind the orbit, are characteristic reptilian features.

The ribs have capitula and tubercula, and sternal and abdominal ribs occur. The sternum has a well-developed keel, and the scapula and coracoid are large and bird-like. There are no clavicles or interclavicle.

The anterior limbs are modified to form wings by the great elongation of the fifth digit, to which a membrane was attached. The second, third and fourth digits are clawed and are not elongated in the way that they are in bats. The pollex, if present at all, is quite vestigial.

The pelvis is weak and small, and though the ilia are produced both in front of and behind the acetabula, in other features the pelvis is not bird-like. The ischia are short and wide, and the pubes are represented only by the pre-pubes. The posterior limbs are small and the fibula is much reduced. The pes is quite reptilian in type, and has five separate slender metatarsals. The two best known genera are Pterodactylus, in which the tail is short, and Rhamphorhynchus, in which it is long. The Pterosauria are found throughout the Jurassic and Cretaceous formations in both Europe and North America.