226 VERTEBRATE ANIMALS OF THE UNITED STATES 



nictitating membrane. The tympanum is at the surface of the body, 

 but in some species is hidden. The neck is long and flexible and has 8 

 vertebras, and together with the head can usually be retracted within 

 the shell. Two pairs of stout, pentadactyle limbs are generally present, 

 the toes of which are more or less webbed in the aquatic turtles. In the 

 marine turtles the limbs are flippers, with a reduced number of toes. 

 The tail is usually short and thick, and in many species ends with a 

 claw. The skin covering the soft portion of the body is usually pro- 

 vided with scales. 



a b 



Fig. 128. — -Carapace (a) and plastron {b) of Chrysemys picta marginala, to show the 

 plates: A, abdominal; An, anal; C, costal or lateral; D, dorsal or vertebral; G, gular; H, 

 humeral; A/, marginal; N, nuchal; P, pectoral; Pr, preanal {from Hurler). 



Habits and Distribution. — Most species of turtles are more or less 

 aquatic in their habits. The land turtles, however, live exclusively 

 on the land, and will often drown if thrown into the water; and all of 

 them, even the giant marine turtles whose limbs are flippers, come on to 

 land to lay their eggs. 



Turtles are noted for their tenacity of life. Many species can 

 remain submerged in the water several hours without drowning. Some 

 species have true water respiration. Species of the TrionychidcB, for 

 instance, take water into the mouth and expel it regularly, when sub- 

 merged, and a number of species have rectal respiration. 



Turtles feed on vegital and animal food. The land turtles feed 

 principally on the former, but also eat grubs, worms and other small 

 animals they can catch. The aquatic turtles usually feed mainly on 

 animal food, which they devour under the water. The snappers and 



