5° 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 vertebra 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- 
Fic. 46.—Tails of fishes. A, young Amia; skeleton (homocercal); B, diphycercal; C, 
heterocercal; D, homocercal; hk, hypurals; ”, notochord; s, spinal cord. 
cercal 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 
hzmals into larger plates (hypurals), while the lower lobe of the tail 
grows out to equal the other. 
