ANATOMY OF VERTEBRATES. 17 



vertebrae, the sternum, and dermal bones, an expanded 

 thoracic-abdominal case, into which the limbs, tail, and, usually, 

 the head, can be withdrawn ; sacrum of more than two vertebrae ; 

 no teeth ; external nostril single, a cavum tympani ; body covered 

 by horny scales in most ; ventricle of heart single. 



Limbs natatory. Genus Chelone. (Turtles.) 



T . , -..-.. f Trionux. (Mud turtles.) 



-Limbs amphibious. { ,-, J /rr i \ ' 



1 [ Jbmys. (lerrapenes.) 



Limbs terrestrial. Testudo. (Tortoises.) 



Order XIX. LACERTILIA. 



Vertebra? procoelian, with a single transverse process on each 

 side, and with single-headed ribs ; sacral vertebra? wanting, or 

 not exceeding two ; two external nostrils ; eyes with moveable 

 lids ; body covered by homy, sometimes bony, scales. 



Limbs natatory, no sacrum. Ex. Mosasaurus. (Extinct.) 

 Limbs ambulatory, a sacrum. Ex. Lacerta, L. 

 Limbs abortive, no sacrum. Ex. Anguis. 



*r>* 



Order XX. OPHIDIA. 



Vertebras very numerous, procoelian, with single-headed hollow 

 ribs ; no sacrum ; no visible limbs ; two external nostrils ; no 

 cavum tympani ; eyeball covered by an immoveable transparent 

 lid. Body covered by horny scales. Teeth anchylosed to jaw. 



Order XXI. CROCODILIA. 



Teeth in a single row, implanted in distinct sockets ; 

 external nostril single and terminal or sub-terminal. Anterior 

 trunk vertebras with par- and di-apophyses, and bifurcate ribs ; 

 sacral vertebras two, each supporting its own neural arch ; this 

 arch usually articulated by suture. Tail long, vertically com- 

 pressed ; feet short, webbed. Skin protected by bony, usually 

 pitted, plates. Ventricle of heart double. 



Suborder Amphiccelia (vertebras cupped at both ends). 



Ex. Teleosaurus. 



Suborder Opisthocmelia (vertebras convex in front, concave 

 behind). Ex. Streptospondylus. 



Suborder Proc(ELIA (vertebras concave in front, convex 



behind). Ex. Crocodilus. 

 vol. i. C 



