Page:The Osteology of the Reptiles.pdf/117

Rh are known only among amphibians. Certain ancient fishes (Eurycormus), it is true, with dorsal embolomerous vertebrae, have in the tail pseudo-rhachitomous vertebrae, composed of two half-disks, the one with its base below, the intercentrum, the other with it above, the undivided pleurocentrum.

The evolution, then, of the holospondylous reptilian vertebra from the temnospondylous amphibian vertebra seems clear: by the simple increase in size of the notochordal centrum and the progressive decrease of the intercentrum to a wedge-shaped, subvertebral bone, and its final loss everywhere in the column save in the atlas and chevrons of the tail; and thus the term hypocentrum becomes purely a synonym of the earlier term intercentrum. The retrogression of the disk-like pleurocentrum into the paired pleurocentra of the Rhachitomi, is paralleled by the separation of the primitively single intercentrum into pairs, observed in Procolophon, many turtles, and some plesiosaurs.

The number of vertebrae in the neck or cervical region of reptiles is not always easily determinable. In those reptiles having a sternum, the first rib attached to it definitely determines the beginning of the thorax. The distinction is almost as definite in those in which there is a change in the articulation of the rib from the centrum to the arch, as in the Sauropterygia and Archaeosauria. But the early reptiles had no sternum, and free ribs were continuous from the atlas to the sacrum without change in their mode of articulation. In such, the changes in their shapes, with other modifications, may indicate approximately the beginning of the dorsal series. Better evidence, however, is found in the position of the pectoral girdle as found in the rocks.

The number is very variable, more so than that of the dorsal vertebrae. The Cotylosauria, like the Temnospondyli, have but one or two vertebrae which may properly be called cervical, since the pectoral girdle is almost invariably found lying immediately back of the skull, the front end of the interclavicle, indeed, between the angle of the jaws. Primitive reptiles, then, like their immediate ancestors, the Stegocephalia, had practically no neck, and but little motion of the head in a lateral direction.