In Reptilia the centra of the vertebrae are usually procoelous, though there are a few examples, such as the archaic Tuatera lizard (Sphenodon), in which the amphicoelous arrangement per sists. There are several cervical vertebrae instead of one, which is all the amphibians have. The odontoid bone is usually separate both from the atlas and axis. Two sacral vertebrae (i.e., ver tebrae articulating with the ilium) are generally present instead of the one of the Amphibia, but they are not fused together as in mammals. In the tail region haemal arches are often found en closing the caudal artery and vein as in urodele amphibians ; in some species these are separate and are then spoken of as chevron bones. In the Crocodilia intervertebral• disks first appear. Ribs are present in the cervical, thoracic and lumbar regions, and in the Chelonia (tortoises) the cer vical ones blend with the verte brae as they do in higher forms.
In crocodiles a definite vertebrar terial canal is established in the cervical region which hencefor ward becomes permanent. The shafts of the ribs are sometimes all in one piece as in snakes or they may be developed by three separate centres as in Sphenodon with intervening joints.
In Crocodilia and Sphenodon there are spurs from each thoracic rib which overlap the next rib behind and are known as uncinate processes; they are developed in connection with the origin of the external oblique muscle of the abdomen and are very constant in birds. The ventral elements of some of the hinder ribs are found in the Crocodilia lying loose in the myocommata of the rectus and obliquus internus (in scriptiones tendineae) and are known as abdominal ribs, while the sacral vertebrae articulate with the ilium through the inter vention of short rods of bone, sometimes called pleurapophyses, which are no doubt sacral ribs. The sternum of reptiles is a broad plate of cartilage which may be calcified but is seldom converted into true bone ; it always articulates with the cora coids (see section Appendicular) anteriorly and with a variable number of ribs laterally and posteriorly. It should not be con founded with the dagger-shaped inter clavicle which, like the clavicles, is a membrane bone and overlaps the sternum ventrally.
In birds the characteristics are largely reptilian with some specialized adaptations to their bipedal locomotion and power of flight. One effect of this is that the two true sacral vertebrae become secondarily fused with the adjacent lumbar, caudal and even thoracic, and these again fuse with the ilium so that the posterior part of a bird's trunk is very rigid. The neck, on the
other hand, is very movable and the centra articulate by means of saddle-shaped joints which give the maximum of movement combined with strength (see JOINTS). The caudal vertebrae are fused into a flattened bone, the pygostyle, to support the tail feathers. In the fossil bird Ar chaeopteryx the centra are am• phicoelous and the long tail has separate caudal vertebrae. The ribs are few and consist of dorsal and ventral parts; the former al most always have uncinate pro cesses. Free cervical ribs are often present and Archaeopteryx possessed abdominal ribs. The sternum is very large and in fly ing birds (Carinatae) has a median keel (carina) projecting from it, while the non-flying, ostrich-like birds (Ratitae) have no such structure.
In Mammalia the centra articulate by means of the inter vertebral disks and it is only in this class that the epiphysial plates appear though these are absent in the Monotremata (duck-mole etc.) and Sirenia (sea-cows). The cervical vertebrae are with a few exceptions (two-toed and three-toed sloths and the manatee or sea-cow) always seven in number, and some, usually all, of them have a vertebrarterial canal in the transverse process. In some of the Cetacea they are fused together. In the Ornitho rhynchus the odontoid is a separate bone, as in many reptiles, but this part includes the facets by means of which the axis and atlas articulate. The thoracic vertebrae vary from ten in some of the whales and the peba armadillo to twenty-four in the two toed sloth, though thirteen or fourteen is the commonest number. In the anterior part of the thoracic region the spines point back ward, while in the posterior thoracic and lumbar regions they have a forward direction. There is always one spine in the pos terior thoracic region which is vertical, and the vertebra which bears this is known as the anticlinal vertebra. The lumbar ver tebrae vary from two in the Ornithorhynchus and some of the armadillos to twenty-one in the dolphin, the average number being probably six. Both the mammillary and accessory tubercles are in some forms greatly enlarged. It is usually held that the former are morphologically muscular processes while the latter represent the transverse processes of the thoracic vertebrae. In the American edentates additional articular processes (zygapo physes) are developed, so that these animals are sometimes divided from the old-world edentates and spoken of as Xenarthra.