There are in living Reptilia two sacrals, but the number may be greater among extinct forms, e.g. 3-6 in Pterodactyles, 4-6 in some Deinosauria. In the Ichthyosauria the atlas and axis resemble the remaining vertebrae in their general form. They are often anchylosed in Plesiosauria and in Cretaceous Pterodactyles. The atlas in most Reptilia consists of a ventral body and two supero-lateral arches, to which is added in the Crocodilia a superior or dorsal membrane bone: and the axis has a well-developed odontoid process united to it by cartilage, by suture in Crocodilia, or by anchylosis as in Lacertilia. The caudal vertebrae in many Lacertilia have a transverse unossified septum through which the tail is apt to split.
Ribs are always present. Their proximal ends though possessing two articular surfaces may be simple, e. g. Ophidia, Lacertilia, or deeply divided, e. g. Crocodilia. The cervical ribs of Chelonia are simple nodules which are either fused to the vertebrae, or united to them by suture. Their dorsal ribs are attached between the centra of adjoining vertebrae. The dorsal ribs when a sternum is present may be divided into a vertebral, intermediate, and sternal section, e. g. Crocodilia, many Lacertilia; or into vertebral and sternal only: or, when the sternum is either absent (Ophidia, some serpentiform Lacertilia, Chelonia, Plesiosauria, Ichthyosauria) or rudimentary, they remain undivided. In the Chamaeleons and Gecko, some of the post-sternal dorsal ribs are continuous ventrally. The sacral ribs are usually massive and expanded distally, the caudal anchylosed to the vertebrae. So-called abdominal ribs are found as ossifications of the intermuscular septa (Crocodilia) or of the subcutaneous fibrous tissue (Hatteria). The sternum is present but is very small and rudimentary in some serpentiform Lacertilia: in some other Lacertilia, e. g.
Acontias meleagris, its right and left halves are stated to remain separate: in others again its bilateral origin is evidenced by a median fontanelle, and by the prolongation backwards, as occurs also in Crocodilia, of two xiphisternal horns with which a certain number of ribs articulate. It is generally more or less rhomboidal in shape, cartilaginous and partially ossified in substance. In many Deinosauria, e. g. Brontosaurus, Iguanodon, there are two sternal bones which were either joined by cartilage or had each a separate cartilaginous border. The sternum of Pterodactyles is well ossified, the cartilage probably replaced by membrane bone, and there is a median keel to the ventral surface.
The shoulder-girdle and fore-limb are absent in Ophidia, but in serpentiform Lacertilia the former is present when the limb is absent. There is a scapula and coracoid, the latter articulating with the sternum. The scapula generally possesses a dorsal cartilaginous but partially ossified supra-scapula, the coracoid a similar but ventral epicoracoid. In many Lacertilia, where the bones are broad, there are membranous tracts forming fenestrae, especially in the coracoids: and the epicoracoids overlap one another anteriorly. The scapula and coracoid are anchylosed in the gigantic Pteranodontia among Pterodactyles, and the scapula articulates with the neural spines of several anchylosed vertebrae, a unique feature among Ver-tebrata. The clavicle is absent in Chelonia, Crocodilia, Chamaeleons, etc, in some Deinosauria, if not in all, and in Pterodactyles. It is continuous with the scapula in Chelonia, forming a large stout bone: separated from the scapula and slender in other groups. There is an interclavicle or episternum in Crocodilia: it is large and T-shaped in Lacertilia and Ichthyosauria, and the two clavicles are closely connected with its anterior border.
There is no trace of a pelvis nor of hind-limb in the majority of Ophidia, but the Typhlopidae, Tortricidae and Pythonidae have both a rudimentary ischio-pubis and limb. In the serpentiform Lacertilia there is a rudimentary ilium. Other Reptilia have a pelvis composed of ilium, pubis and ischium. The iliac axis slopes forwards and downwards, and the bone usually extends backwards, i. e. behind its point of union with the sacral ribs, but in the Crocodilia, and especially in many Deinosauria, e. g. Iguanodon, Stegosaurus, it has a forward extension indicated in many Lacer-tilia by a process (processus acetabularis). The acetabulum is formed by the three bones jointly, except in the Crocodilia where the pubis is excluded from it. The centre of this cavity is fibrous in Crocodilia and many Deinosauria. As a rule, there is a pubic and ischial symphysis, but the former is wanting in many Deinosauria, e. g. Iguanodon. The pubis varies in shape and size. In the Stegosauria and Omithopoda among Deinosauria it has a peculiar shape; one portion, the post-pubis, being long and slender and parallel to the ischium which, in these two groups, is strongly inclined backwards as in Aves; the other, the praepubis, projecting downwards and forwards.
The latter is probably the homologue of the pectineal tubercle. There is no evidence that the two portions represent two distinct bones (see p. 65). There is a large obturator foramen between the pubis and ischium of the same side, but the obturator nerve either perforates the pubis itself (Lacertilia) or the fibrous membrane closing the foramen. In land and fresh-water Chelonia, the pubes and ischia touch one another in the median line; but in the marine Chelonia, in the Crocodilia and Lacertilia, the two obturator foramina are only separated medianly by a strong fibrous band extending from the former to the latter. There is a small epipubic cartilage in many Lacertilia and Chelonia, and in many of the former an os cloacae extends backwards from the ischial symphysis below the cloaca.
The Ophidia with a few exceptions (supra) are limbless. Certain Lacertilia (most Amphisbaenoidea, some Brevilinguia, e.g. Anguis) are in the same condition. Others possess rudimentary limbs with or without a reduced number of toes. In other Reptilia limbs are well developed. They are short and paddle-like in Plesiosauria and Ichthyosauria, and in the latter the digits often bifurcate, and there are rows of extra ossicles at their margins. The Omithopoda among Deinosauria appear to have walked erect, and their fore-limbs are short. The Pterodactyles have the fifth finger elongated to support the wing membrane, and the fore-limb is large and powerful in them. But as a rule, the limbs are small in comparison with the body. There is much variety in the conformation of the carpus and tarsus, of the digits and their phalanges, but the following points may be noted. The carpus of some Chelonia possesses all the typical ossicles separate. In the tarsus the ankle-joint comes to lie between the proximal and distal tarsals in many Chelonia, the Lacertilia, the Omithopoda among Deinosauria: and in the Omithopoda the astragalus has an ascending process more or less developed and at least applied to, or in some instances perhaps fused with the tibia, thus approximating to Aves. The Crocodile has a well-developed calcaneal process.