96 THE OSTEOLOGY OF THE REPTILES 



I filer centra. 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 inter centrum (Fig. 

 76 e). Intercentra had previously long been known as "interverte- 

 bral" or " sub vertebral wedge-shaped bones," but their significance 

 was ill understood. With the more complete ossification of the ver- 

 tebral 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, hav- 

 ing 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 noto- 

 chordal 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 for- 

 ward on the preceding centrum, either loosely attached or coossified 

 with an exogenous outgrowth, forming with it a functional hypa- 

 pophysis. Where they occur between the centra they may be elon- 

 gated 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 a-c) and rhachitomous (Fig. 76 d). The former 

 are known in only a few amphibians, from the Mississippian, Penn- 



