REPTILES 235 



the circumference of the shell by a series of marginal dermal bony plates; 

 in the plastron they consist of exclusively dermal plates, there being 

 no sternum. The carapace and the plastron are more or less firmly 

 united by a wide bridge on each side. 



The head is a very solid and compact structure, and is often covered 

 with scales. The jaws are toothless and are covered by a horny sheath 

 which forms a cutting edge. The Trionychidcd are the only turtles which 

 have fleshy lips. The eye has an upper and a lower lid and also a 

 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 

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

 the shell. Two pairs of stout, pentadactyle hmbs are always present, 

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

 marine turtles the hmbs 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 provided with 

 scales. 



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

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

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

 them, even the giant marine turtles whose Hmbs are fhppers, 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 

 submerged, 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 

 soft-shells are fierce beasts of prey, which eat large numbers of fish, 

 water birds and other animals which frequent the water. 



Turtles usually breed in the late spring or early summer. All are 

 oviviparous; the eggs are buried by the female in the sand or earth and 

 are incubated by the heat of the sun. 



About 260 species of turtles are known, of which about 50 species 

 are found in the United States; these are grouped in 6 families. 



On the Identification of Turtles. — The epidermal, horny plates 

 forming the outer covering of the carapace fall into three groups, the 

 dorsal or vertebral, the lateral or costal and the marginal plates (Fig. 



