50 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



dens or odontoid process) on which the atlas turns. Development 

 shows that this dens is the centrum of the atlas which has separated 

 from its own verfebra and has fused to that of the axis. 



In a few reptiles and possibly some mammals a so-called proatlas occurs as a 

 plate or pair of plates (fig. 45) of bone between the atlas and the skull, in the posi- 

 tion of a neural arch. It is not certain whether this is the remains of a vertebra 

 which once occupied this position, or is a new formation. Nor has it been settled 

 whether the atlas of the amphibians is homologous with that of mammals. 



In cyclostomes, fishes and aquatic urodeles the posterior end of the 

 vertebral column is concerned in the formation of the caudal fin, 

 which presents three modifications. The most primitive is the diphy- 



FIG. 46. Tails of fishes. A, young Amia; skeleton (homocercal) ; B, diphy cereal ; C, 

 heterocercal; D, homocercal; h. hypurals; , notochord; s, spinal cord. 



cereal tail in which the vertebral column runs straight to the end of the 

 body, the fin being developed symmetrically above and below it. This 

 is found in the young of all fishes and in the adult cyclostomes, dipnoans, 

 many crossopterygians and urodeles. In the heterocercal tail, which 

 occurs in elasmobranchs and ganoids, the axis bends abruptly upward 

 near the tip, and while retaining the caudal fin of the diphycercal stage, 

 has a second, smaller lobe developed below, giving the whole an unsym- 

 metrical appearance. In the homocercal tail, which occurs in Amia 

 and all teleosts since the cretaceous, there is the same upward bend to 

 the vertebral column, but symmetry is restored externally by the re- 

 duction of the neural arches and the development and fusion of the 

 haemals into larger plates (hypurals), while -the lower lobe of the tail 

 grows out to equal the other. 



