58 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



undergo more or less calcification (p. i), the lime being either deposited in con- 

 centric rings around the notochord (cyclospondylous vertebrae) or in radiating 

 plates (asterospondylous, fig. 53). 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.) diplo- or polyspondyly (p. 53) occurs in the tail, and in 

 the holocephali the centra are replaced by numerous rings of cartilage. In 

 skates and in Chimcera there is a true joint between the skull and the column, 

 but in the sharks the anterior vertebrae are fused together and to the skull. 



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 Amia and Lepidosteus, the latter 

 having opisthoccele vertebrae, a condition not reappearing until the amphibians, 

 as all other fishes in which centra are developed have amphicoelous vertebrae. 



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. 54), so that the section presents a radiate figure as in the 

 asterospondylous sharks. Some teleosts have zygapophyses and a few genera 

 have transverse processes on some of the vertebrae. 



The dipnoans, so far as ossification of the vertebrae is concerned, are on a par 

 with the cartilaginous ganoids. There are no centra and the notochord grows 

 throughout life. 



AMPHIBIA, except the legless forms (gymnophiona, and aistopoda of the 

 stegocephals) , have caudal, sacral, and trunk regions, and a single cervical verte- 

 bra, the sacrals being single except in a family of extinct anurans. Zygapophy- 

 ses and both kinds of transverse processes may be present. 



The stegocephals had the greatest range of vertebral structure, rhachito- 

 mous, embolomerous, and amphicoelous types occurring, the first two even in 

 the same individual. Phyllospondylous vertebrae (fig. 47) only in the fossil 

 Branchiosauridae. 



The caecilians have a very large number (up to 275) of amphicoelous verte- 

 brae in correlation with the snake-like body form. The perennibranchs, dero- 

 tremes and many salamandrina are amphicoelous; the rest of the urodeles are 

 opisthocoelous. 



The anura, as a rule, have proccelous vertebrae, but a few genera have them 

 opisthoccele. All recent species have eight presacral vertebrae, but there were 

 nine in the tertiary forms. A striking feature is the fusion, in the adult, of all 

 of the caudal vertebrje into the well-known rod, the coccyx or urostyle. 



REPTILES always have the vertebrae ossified, although remnants of the 

 notochord may persist in the centra, of which all types, amphi-, pro-, opistho- 

 coelous and flat occur in the group. In lizards, snakes and dinosaurs the articu- 

 lation between the successive vetebrae is strengthened by zygantra and zygo- 

 sphenes, a cavity on one vertebra into which a projection from the next fits. 

 In the existing species there are never more than two sacral vertebrae, but the 

 pterosaurs had from three to seven, while in the dinosaurs there might be ten, 

 all being co-ossified when there were more than three. 



Little is known of the theriomorph vertebrae, except that some species had 



