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96 Intercentra. The earliest reptiles probably all have a small or vestigial, more or less wedge-shaped bone intercalated between the adjacent ventral margins of the centra throughout the column, to which Professor Cope in 1878 gave the name intercentrum (Fig. 76 ). Intercentra had previously long been known as "intervertebral" or "subvertebral wedge-shaped bones," but their significance was ill understood. With the more complete ossification of the vertebral centra they began to disappear in the dorsal region, in early or middle Permian times, but have remained to modern times in the gecko lizards and in Sphenodon. They have persisted in nearly all reptiles in the tail as the chevrons, and more or less in the neck, having been entirely lost as simple intercentra only in the crocodiles and a few other reptiles. The intercentrum of the first vertebra has remained functional in all Amniota as the basal piece or "body" of the atlas.

Intercentra are characteristic of deeply amphicoelous or notochordal dorsal vertebrae, that is, in the more primitive vertebrae, and never occur in procoelian, amphicoelian, or opisthocoelian reptiles. They occur in many procoelous lizards throughout the neck, often in their normal places between the centra but frequently shifted forward on the preceding centrum, either loosely attached or coössified with an exogenous outgrowth, forming with it a functional hypapophysis. Where they occur between the centra they may be elongated into false hypapophyses. A similar condition is known in some Chelonia on the first two to four vertebrae, where they are usually paired. Double intercentra have also been observed in the anterior vertebrae of Procolophon, a cotylosaur, and in the young of certain plesiosaurs. In the Ichthyosauria, though the centra are deeply biconcave, only two to four intercentra have been observed. They have also been found in the anterior vertebrae of some plesiosaurs.

It is now universally believed that the undivided or holospondylous vertebrae of reptiles were evolved from divided or temnospondylous vertebrae of the Stegocephalia. It was Cope who first recognized the identity of the parts and his views are now generally accepted, though not by all.

Temnospondylous vertebrae are of two kinds, called by Cope embolomerous (Fig. 76 ) and rhachitomous (Fig. 76 ). The former are known in only a few amphibians, from the Mississippian,