VERTEBRAL COLUMN, RIBS, AND STERNUM 305 



has occurred, and the vertebras of these animals have centra 

 which correspond to pleurocentra. In consequence of this, 



>M 



Fig. 154. — Vertebral columns of A, a bony fish (tunny) ; B, crocodile (in 

 the tail-region) ; C, Sphenodon (trunk region). 



c, centrum ; cb, chevron-bone ; ha, hasmal arch ; he, hypocentrum ; 

 na y neural arch ; pc, pleurocentrum ; pz, prezygapophysis ; r, rib ; tp y 

 transverse process. 



the haemal arches in amphibia are always attached to the centra 

 themselves ; whereas in the amniota, when they occur (mostly 

 in the tail-region), the haemal arches are attached to separate 



