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.