160 REPTILES. 



No traces of a true sternum have been discovered 

 (Huxley). The plastron consists of membrane bones, of 

 which there are usually nine pieces four pairs and a 

 single symmetrical median piece. These correspond 

 neither in number nor position with the overlying 

 dermal plates. 



The skull is more compact than that of the other Rep- 

 tiles. There are no teeth, but the jaws are encased in 

 horny sheaths, usually with sharp cutting edges; the eye 

 is furnished with two lids and a nictitating membrane as 

 in the Birds; the tympanic membrane is always present, 

 although sometimes hidden by the skin. Respiration is 

 effected by swallowing air. 



The order Testudinata is divided by Prof. Agassiz into 

 two sub-orders: AMYD^E, comprising the Land and 

 Fresh Water Turtles, with retractile feet that may be 

 used for walking; and CHELONII, the Sea Turtles, with 

 flipper -like feet used chiefly for swimming. Of the 

 latter, several species occur on our coast, but we here 

 omit them. 



FAMILY LXVI. TESTUDINID^. 



(TJie Land Tortoises.) 



Carapace strong, thick, ovate, generally very convex 

 and falling off abruptly at both ends; caudal shields 

 united into one; plastron very broad, covering the 

 whole under surface, the anterior part sometimes mov- 

 able on a transverse hinge. Legs and feet club-shaped; 

 toes firmly bound together by the integument, only the 

 blunt claws being exserted. 



Herbivorous Turtles, entirely terrestrial, inhabiting the 

 warmer parts of both continents; about twenty species 

 are known. 



