ANATOMY OP VEKTEBKATES. 17 



vei'tebrre, the sternum, and dermal bones, an expanded 

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

 tlie head, can be withdrawn ; sacrum of more than two A'crtcljra; ; 

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

 1 ly horny scales in most ; ventricle of heart single. 



Limbs natatory. Genus Chclone. (Turtles.) 



T . 1 , ., . f Trionyx. (Mud turtles.) 



Ijimbs amiilubious. \ r, "^ /m \ 



^ \ hniijs. (lerrapenes.) 



Limbs terrestrial. Teshido. (Tortoises.) 



Order XIX. LACERTILIA. 



Vcrtel3ra3 prococlian, with a single transverse pu'occss on each 

 side, and Avith single-headed ril)s ; sacral vertebrae wanting, or 

 n(.)t exceeding two ; two external nostrils ; eyes with moveable 

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



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

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

 Liniljs abortive, no sacrum. Ex. Anguis. 



Order XX. OPHIDIA. 



Vertebra; very numerous, procoelian, with single-headed hollow 

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

 ca^'um tympani ; eyeball covered by an immoveable transparent 

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



Order XXI. CROCODILIA. 1'^- ' ' ' 



Teeth in a single row, implanted in distinct sockets ; 

 external nostril single and terminal or sub-terminal. Anterior 

 trunk vertebra; Avitli par- and di-apophyses, and bifurcate ril^s ; 

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

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

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

 pitted, plates. Ventricle of heart double. 



Suborder Aimpiiiccelia (vertebra; cupped at both ends). 

 Ex. Teleosaurus. 



Suborder OriSTiiocfELiA (vertebra; convex in front, concave 

 behind). Ex. Streptospondylus. 



Suborder Peoccelia (vertebra; concave in front, convex 



behind). Ex. Crocodilus. 

 VOL. I. c 



