46 COMPARATIVE ANATOMY 



comes entirely aborted and replaced by bony tissue. This stronger 

 and more solid ossification of the whole skeleton forms a character- 

 istic difference between the Ichthyopsida on the one hand and 

 the Amniota on the other. As a rule the vertebrae of Reptiles 

 become definitely articulated with one another, and are of the 

 proccelous type : the above-named forms, with intervertebral re- 

 mains of the notochord, form an exception to this rule. In Croco- 

 diles fibro-cartilaginous intervertebral discs or menisci occur between 

 the centra (Fig. 35). 



In Crocodiles the vertebra? 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 vertebrae even in the same 

 individual procoelous, opisthocoelous, biconcave, and even biconvex centra,, 

 with intervertebral discs, may occur ; while the thoracic and lumbar vertebrae 

 have flattened faces, and are firmly united together by cartilage. 



In the Jurassic Ichthyosaurus and Eosaurus the centra were short and 

 deeply biconcave, like those of Fishes, and the arches were connected with 

 them by cartilage and connective tissue ; as a sacrum was absent, only a 

 precaudal and a caudal region can be recognised. In Plesiosaurus, Plio- 

 saurus, Nothosaurus, Simosaurus, the Anomodoiitia and others, the centra 

 were also biconcave or flattened. 



What has been said as to the classification of the vertebrae into 

 different regions in Urodeles, as well as to the presence of the 

 various processes, usually applies here also to a still greater extent.. 

 Except in limbless form, there are always several cervical vertebrae 

 instead of a single one : there are also typically at least two sacral 

 vertebrae. The two first cervical vertebrae become differentiated 

 to form an atlas usually consisting of three pieces, and an axis- 

 with an odontoid bone (Fig. 35, and comp. p. 45). l 



The neural spines vary in size, and transverse processes arise 

 from the centra themselves or close to them. Lower arches,, 

 attached intercent rally (chevron bones) are present in the tail in 

 Lizards, Crocodiles, and some Chelonians ; and besides these,, 

 median inferior processes of the centra themselves (? intercentra) are 

 seen in many of the vertebrae of Lizards, Crocodiles, and Snakes., 

 and in the latter paired processes partly enclose the caudal vessels. 

 The arches in Snakes, Lizards, and Chelonians become united 

 with the centra by synostosis, while in Crocodiles they remain, 

 separated from them by sutures (Fig. 35). 



In consequence of the absence of a pectoral arch, the vertebral 

 column of Snakes and Amphisbsenians, like that of Csecilians,. 

 consists of trunk and caudal vertebrae only. The vertebral column 

 of Chelonians deserves particular notice as a large portion of it 

 becomes anchylosed with the dermal bones of the carapace (p. 33, 

 Fig. 21), and is thus rendered immovable. 



1 The odontoid bone corresponds morphologically to a part of a centrum of the 

 atlas. A so-called pro-atlas the remains of a vertebra situated between the 

 skull and atlas proper is present in the Crocodilia (Fig. 35), Hatteria, and 

 Chamselaeo, as well as in many fossil forms. . 



