58 COMPARATIVE ANATOMY 



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, and also typically at least two sacral 

 vertebrae. The two first cervical vertebrae become differentiated 

 to form an atlas bearing a single occipital facet and usually 

 formed of three pieces, and an axis with an odontoid bone 

 belonging morphologically to the centrum of the atlas (cf. p. 57). 



The neural spines vary in size, and transverse processes arise 

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

 chevron bones, corresponding to the intercentra, are present in the 

 tail in Lizards, Crocodiles, and some Chelonians ; and besides 



u 



FIG. 46. ANTERIOR PORTION OF THE VERTEBRAL COLUMN OF A YOUNG 



CROCODILE. 



A , atlas ; Ep, axis, articulating with the atlas at h ; Is, intervertebral disc ; o, 

 " pro-atlas " ; Ob, neural arch ; Po, odontoid bone ; Ps, neural spine ; Pt, 

 transverse process, arising from the base of the arch and articulating with 

 the rib at f ; R l , &, R, ribs ; u, ventral element, and s, arch of atlas ; WK, 

 centrum. 



these, median inferior processes of the centra themselves are seen 

 in many of the vertebrae of Lizards, Crocodiles, and Snakes : 

 in the last mentioned paired processes partly enclose the caudal 

 vessels. The arches in Snakes, Lizards, and usually in Chelonians, 

 become united with the centra by synostosis, while in Crocodiles 

 they remain, at any rate for a long time, separated from them by 

 sutures (Fig. 46). 



In Snakes, Hatteria, and some Lizards (Iguana) extra articular 

 processes (zyyosphenes and zygantra) are developed on the neural 

 arches ; and in the caudal region of Hatteria and Lizards an 

 unossified septum remains in the middle of each centrum (which 

 really corresponds to two primary vertebral elements), so that the 

 tail easily breaks off at these points. When this happens the tail 

 grows again, but true vertebrae are not formed. 



