XII 



PHYLUM CHORDATA 



441 



remnants of the notochord in 

 the inter-central spaces. In 

 most of the others the centra 

 are precocious, a ball-like con- 

 vexity on the posterior surface 

 of each centrum projecting 

 into a cup-like concavity on 

 the anterior face of the next. 



The various regions of the 

 spinal column are well marked 

 in most of the lizards, in the 

 Chelonia, and in the Croco- 

 dilia (Fig. 264). In the snakes 

 and many of the snake-like 

 lizards only two regions are 

 distinguishable pre - caudal 

 and caudal. In the others 

 there is a sacral region com- 

 prising two vertebrae, which 

 have strong transverse pro- 

 cesses for articulation with 

 the ilia. The first and sec- 

 ond vertebrae are always modi- 

 fied to form an atlas and 

 axis. Ribs are developed in 

 connection with all the ver- 

 tebrae of the pre-sacral or pre- 

 caudal region; in the caudal 

 region they are usually replaced 

 by inferior arches. In the fly- 

 ing lizards {Draco) a num- 

 ber of the ribs are greatly 

 produced, and support a pair 



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