ELANOSAURES. OPHIDES. 359 



Platysternon Gray. Sternum small, bifid behind, 

 solid ; head very large, not retractile within the 

 shell; feet with five anterior, and four posterior 

 claws j tail very long, equal to the length of the body, 

 scaly, but not crested. 



P. megacephalus. Gray, Ind. Zool, ii. pi. 62. 



Chelydra Schweigg.* Sternum remarkably narrow, 

 and cruciform ; the plates membranaceous, and more 

 resembling skin than bone ; symphysis very narrow ; 

 feet large and robust ; tail very long, with crested 

 longitudinal plates, like those of the alligators. 

 C. serpentina. Shaw, Gen. Zool. iii. 1. pi. 29. 



Order III. ELANOSAURES, Conybeare. Fish 



Lizards. 



Lacertiform;feetin the form of fins, as in the aquatic tur- 

 tles ; tail short, compressed ; the articulating surfaces 

 of the vertebrae concave ; eyes very large, nocturnal. 



Ichthyosaurus Kcenig. Head large, lengthened ; 

 neck short ; teeth none ; jaw received into a pit at 

 the bottom of those in the other jaw. 

 I. communis Cuv. 



Plesiosaurus Conybeare. Head small ; neck exces- 

 sively long, and composed of numerous vertebrae. 

 P. dolichodeirus. Cony. GeoL Trans, i. pi. 48. 



Saurocephalus Harlan. Teeth like incisors, placed 

 in pits, with a regular hole on the inner edge of the 

 alveola. 



S. lanciformis. Am. Trans, v. pi. 16. 



Order IV. OPHIDES. Serpents. 

 Body covered with scales, and either entirely without 



Trionycidce, but it remains to be determined if any, and what, other of the 

 sub-genera, composing Mr. Gray's genus Chelys (GrifiF. Cuv. p. 14.), really 

 belong to this group, or form part of the Emydce. 



* We regret that the inviolable lavr of priority, without which our no- 

 menclature would be daily changing, obliges us to retain this name instead 

 of the peculiarly expressive one of Emysaurus. 



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