COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The stegocephals had the greatest range of vertebral structure, rhachitomous, 

 embolomerous, and amphiccelous types occurring, the first two even in the same 

 individual. Phyllospondylous vertebrae (fig. 42) are found only in the fossil 

 Branchiosauridae. 



The caecilians have a very large number (up to 275) of amphiccelous vertebrae 

 in correlation with the snake-like body form. The perennibranchs, derotremes and 

 many salamandrina are amphiccelous; the rest of the urodeles are opisthoccelous. 



The anura, as a rule, have precocious 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 verte- 

 brae into the well-known rod, the coccyx or 

 urostyle. 



REPTILES always have the vertebrae ossi- 

 fied, although remnants of the notochord may 

 persist in the centra, of which all types, amphi-, 

 pro-, opisthoccelous and flat occur in the group. 

 In lizards, snakes and dinosaurs the articulation 

 between the successive vertebrae is strengthened 

 by zygantra and zygosphenes, a cavity on one 

 vertebra into which a projection from the next 



FIG. 49. FIG. 50. 



FIG. 49. Cervical vertebra of a bird showing the saddle-shaped articular surface (of) 

 on the centrum, c; cr, cervical rib; nc, neural canal; ns, neural spine; poz, prz, post- and 

 prezygapophyses. 



FIG. 50. Central view of synsacrum and pelvis of hawk (Buteo). il, ilium; is, 

 ischium; p, pubis; pp, pectineal process; s, sacral ribs. 



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 backbone, except that some had persistent 

 notochords, others amphiccelous centra. In the plesiosaurs they were flat, while 

 in the turtles the dorsals are fused and the neural spines are united with the neural 

 plates (p. 41). The other centra vary. Those of the rhynchocephals and most 

 dinosaurs are flat, while snakes and lizards, except the geckos have them proccelous. 

 In the earliest crocodiles' they were amphiccelous, while later they are proccelous or 

 flat, and in the pterodactyls they are proccelous in front, amphiccelous in the tail. 



BIRDS usually have saddle-shaped ends to the centra (the atlas proccelous). 



