32 



WATER REPTILES OF THE PAST AND PRESENT 



Ordinarily the cervical vertebrae differ from those behind them 

 only in the small size or fusion of their ribs; sometimes, however, 

 as in the Protorosauria and Pterosauria, the vertebrae may be much 

 elongated. The dorsal vertebrae of reptiles vary in number from 

 ten in turtles and some dinosaurs to forty-three in Pleurosaurus; 



and under the name dorsal we 

 include the so-called lumbar, as 

 there is seldom any real distinction 

 between the two series, save the 

 smaller size or the co-ossification 

 of the ribs of the latter. 



The sacrum in reptiles primi- 

 tively consisted of a single verte- 

 bra, which bore a large rib on each 

 side for the support of the pelvis. 

 Very early, however, a second or 

 even a third vertebra was added 

 to it from behind. The number 

 two is the rule among reptiles, 

 both ancient and modern; among 

 crawling reptiles the number never 

 exceeds three, but among ambulatory and flying reptiles the num- 

 ber may be as great as in any mammal. 



The number of caudal vertebrae in reptiles is exceedingly 

 variable, from a dozen or fifteen up to a hundred and fifty or more. 

 In snakes but two regions are 

 distinguishable, the caudal 

 and precaudal, and the num- 

 ber altogether may reach 

 nearly five hundred. With 

 the exception of the first 

 few basal caudal vertebrae 

 (pygals) and the minute ones 



at the extreme tip, all caudal vertebrae of reptiles bear a slender, 

 usually Y-shaped bone below in the interval between the centra, for 

 the protection of the vessels and nerves. Because of their shape 

 they have been called chevrons, and are really outgrowths from the 

 intercentra . 



Fig. 1$. — Ophiacodon, a primitive 

 theromorph reptile: proatlas, atlas, 

 and axis, with ribs. 



Fig. i6. — Sacrum of Chelone 



