SKELETON. 143 



arcale). It arises in development from the ventral part of the 

 vertebral bow (p. 137). The axis is in most respects a normal 

 vertebra, but it bears, projecting from the anterior face of its 

 centrum, a more or less cylindrical outgrowth, the odontoid pro- 

 cess ; this is morphologically the centrum of the atlas, whicb 

 has lost its connection with its proper neural arch, and has 

 become secondarily united with the centrum of the second 

 vertebra, forming a pivot about which the atlas turns. 



In crocodiles, Hatteria, and possibly some mammals, a pair of 

 plates (reptiles) or a single plate occurs on the dorsal anterior 

 portion of the neural arch of the 

 atlas. This is the so-called pro- 

 atlas ; but whether this is the last 

 remnant of a vertebra which has 

 otherwise disappeared from be- 

 tween the existing atlas and the 

 base of the cranium cannot yet be 

 definitely decided. Nor is it pos- 

 sible as yet to say whether the 

 only cervical vertebra of the am- FIG. 151. Three anterior verte- 



phibia is homologous with either brae of alligator, a, atlas; e, axis; 

 atlas or axis of the amniotes. , odontoid process; A pro-atlas; r yj 



ribs. 



Ribs. The name rib has been 



applied to two different structures, 1 one appearing in the gan- 

 oids, teleosts, and dipnoi, the other in amphibia and amniotes, 

 and apparently in selachii as far as these latter have ribs. 



The ribs of the fish are the haemal processes of the trunk 

 vertebrae, which, in the region of the body cavity, extend from 

 the vertebral centres towards the ventral surface between the 

 muscles and the ccelomic walls. The transitions from these 

 ribs into the haemal arches can be traced in any fish skeleton. 

 In the caudal region of the urodeles haemal arches comparable 

 to those of fishes are present, and besides these the caudal 

 vertebrae also bear transverse processes which extend directly 

 outwards between the epi- and hypaxial muscles. Following; 



l The view of the ribs adopted here is that which appears to have the better basis. 

 Baur and others hold that ribs are homologous throughout the vertebrates, but their reasons, 

 are not conclusive. 



