SKELETON. 51 
CYCLOSTOMES have a persistent notochord, increasing in size with the 
growth of the animal, and lacking constrictions since no centra are developed. 
In the myxinoids there are neurapophyses and intercalaria developed in the caudal 
region; in the lampreys they occur in the trunk as well. 
FISHES.—In the elasmobranchs the typical vertebra are developed in cartilage, 
with intercalaria in connection with the arches. Usually the centra undergo more 
or less calcification (p. 43), the lime being either deposited in concentric rings 
around the notochord (cyclospondylous vertebrz) or in radiating plates (astero- 
spondylous). In the trunk region each centrum often bears a pair of transverse 
processes with short ribs at their extremities. In a few forms (skates, etc.) 
embolomerism (p. 48) occurs in the tail, and in the holocephali the centra are 
replaced by numerous rings of cartilage. In skates and in Chimera there is a true 
joint between the skull and the column, but in the sharks the anterior vertebra are 
fused together and to the skull. 
Fic. 47. Fic. 48. 
Fic. 47.—Diagrammiatic sections of elasmobranch vertebre. A, B, cyclospondylous; 
C, asterospondylous. 
Fic. 48.—Cross-section of teleost vertebra; bone, black; cartilage, dotted. 
The ganoids vary greatly in vertebral characters, some of the Chondrostei 
having only cartilage and some of the fossil forms lacked centra. On the other 
hand, nearly the whole vertebra is ossified in A mia and Lepidosteus, the latter having 
opisthoceele vertebrae, a condition not reappearing until the amphibians, as all 
other fishes in which centra are developed have amphiccelous vertebre. 
As the name implies, ossification of vertebrae and other parts is common in 
teleosts. The arches are almost always ossified, while the centra may be, or those 
parts directly connected with the arches may remain cartilaginous while the rest 
ossifies (fig. 48), so that the section presents a radiate figure as in the asterospondy- 
lous sharks. Some teleosts have zygapophyses and a few genera have transverse 
processes on some of the vertebre. 
The dipnoans, so far as ossification of the vertebra is concerned, are ona par 
with the cartilaginous ganoids. There are no centra and the notochord grows 
throughout life. 
AMPHIBIA, except the legless forms, have caudal, sacral, trunk, and a single 
cervical vertebra, the sacrals being single except in a family of extinct anurans. 
Zygapophyses and both kinds of transverse processes may be present. 
