52 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 vertebre (fig. 42) are found only in the fossil 
Branchiosauride. 
The cacilians have a very large number (up to 275) of amphiccelous vertebra 
in correlation with the snake-like body form. The perennibranchs, derotremes and 
many salamandrina are amphiccelous; the rest of thé urodeles are opisthoccelous. 
The anura, as a rule, have proccelous vertebre, but a few genera have them 
opisthoccele. All recent species have eight presacral vertebre, but there were nine 
in the tertiary forms. A striking feature is the 
fusion, in the adult, of all of the caudal verte- 
bre into the well-known rod, the coccyx or 
urostyle. 
REPTILES always have the vertebre 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 vertebre is strengthened 
by zygantra and zygosphenes, a cavity on one 
vertebra into which a projection from the next 
Fic. 49. Fic. 50. 
Fic. 49.—Cervical vertebra of a bird showing the saddle-shaped articular surface (af) 
on the centrum, c; cr, cervical rib; mc, neural canal; ms, neural spine; poz, prz, post- and 
prezygapophyses. 
Fic. 50.—Central view of synsacrum and pelvis of hawk (Bufeo). il, ilium; is, 
ischium; ~, pubis; ~p, pectineal process; s, sacral ribs. 
fits. In the existing species there are never more than two sacral vertebre, 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 rhynchocephais 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). 
