166 



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: AMYDJE, comprising the Land and 

 Fresh Water Turtles, with retractile feet that may be 

 used for walking; and CHELONH, 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 LXVL TESTUDINID^E. 



(The 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. 



