CLASS REPTILES. 315 



fatalities are simply occasional. The poisonous type are 

 being rapidly exterminated. The teeth and skin of 

 alligators and crocodiles; the tortoise-shell; the flesh of 

 some turtles embrace about all the direct contributions 

 which the class makes to humans. 



321. Definition of the Reptilia. The reptiles agree in 

 breathing by means of lungs throughout life; in having 

 scales of horny or bony material developed in the skin; 

 in producing eggs of considerable size; and in being cold- 

 blooded. 



322. Key to the Principal Orders of Reptiles. 



With a box-like covering . . . .Order Chelonia (turtles). 



Soft-shelled ; aquatic ; plates of carapace not completely bony, 

 covered with a leathery skin, 



Soft-shelled turtle (several species). 



Hard- shelled ; land or water Testudinidae. 



Land forms; can withdraw completely into box, and close 



flaps of plastron * Box-tortoise. 



Water forms : 



Head large ; not withdrawn into carapace ; given to snap- 

 ping its beak in defense . Snapping turtles (several species) . 

 Head and neck not so long ; plates of carapace dark brown 

 bordered with yellow, 



Painted, or mud-turtles (several species). 

 Without box-like covering. 



Body long and slender; no legs Order Ophidia. 



Head flat and triangular, with small scales; tail blunt; 



venomous Crotalidae. 



Examples: Rattlesnake; copperhead; water moccasin. 

 Head tapering into body and covered with plates; tail more 



tapering; non- venomous Colubridae. 



Examples: Grass snake; garter snake; water snakes; 

 spreading adder; black snake. 

 Body lizard-like, with four legs. 



Small ; cloaca opening by a transverse slit . Order Lacertilia. 



With wide body ; spines on head Horned toad. 



With slender body. 



Body gray, with undulating, dark transverse bands, 



