VERTEBRAL COLUMN 57 



by means of lateral facets with the two condyles of the skull, and 

 also, in Urodeles, with the base of the latter by means of a projec- 

 tion, of varying size and form, the so-called " odontoid " process ; 

 thus a freer movement between the skull and vertebral column is 

 rendered possible. This vertebra, however, is not homologous with 

 the first vertebra (i.e. the atlas) of the higher Vertebrates, as is 

 demonstrated by a study of its development, which shows that the 

 real atlas, with the exception of the part which forms the " odon- 

 toid," loses its individuality as a separate mass, and becomes 

 united with the occipital region of the skull. 



The number of vertebras present in Urodeles is inconstant, and 

 varies greatly : it may reach to nearly 100 (Siren), and in Cascilians 

 may be very much greater (up to 275). 



Reptiles. In many fossil Reptiles (Theromorpha, Ichthyo- 

 sauria, &c.) the centra were biconcave, and this primitive form, 

 with an intervertebrally expanded notochord, is retained in the 

 Ascalabota amongst existing forms : the Rhynchocephali also 

 possess amphicoelous vertebrae, but intercentral fibro-cartilagin- 

 ous discs occur in their existing representative, Hatteria. A 

 primitive character of the Rhynchocephalian vertebral column is 

 seen in the retention throughout of the primary components of the 

 centra as distinct elements, wedge-shaped intercentra being inter- 

 calated between the centra proper or pleurocentra : in the majority 

 of Lacertilia intercentra also occur, but are usually only recognis- 

 able in the neck and tail ; in Chelonians a few intercentra are 

 present in the neck region. A pair of elements interposed between 

 the upper part of the first vertebra (atlas) and the skull in 

 Crocodiles, usually known as the " pro-atlas " (Fig. 46), which is re- 

 presented also in Hatteria, Chameleons, and many fossil forms, corre- 

 sponds to a disconnected pair of " supra-dorsal " elements (p. 49). 



In the majority of Reptiles, the notochord remains expanded 

 longer in the intravertebral regions than intervertebrally, but in 

 the adult it becomes entirely aborted and replaced by bony tissue. 

 This stronger and more solid ossification of the whole skeleton 

 forms a characteristic difference between the Ichthyopsida on the 

 one hand and the Amniota on the other. As a rule the centra 

 of Reptiles are of the procoelous type and become definitely articu- 

 lated with one another : the forms with intervertebral remains of 

 the notochord and those with fibro-cartilaginous intervertebral discs, 

 (e.g. Crocodiles) form an exception to this rule. In Crocodiles the 

 vertebras are mostly procoelous, an exception being seen in the two 

 sacrals and first caudal. In Chelonians there is great variation in 

 the form of the individual centra of the cervical vertebras even in 

 the same individual procoelous, opisthocoelous, biconcave, and even 

 biconvex centra, with intervertebral discs, may occur ; while the 

 thoracic and lumbar vertebra- have flattened faces, and are firmly 

 united with one another by cartilage, and also with the 

 (p. 43). 



