REPTILES 227 



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 245 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. 

 128). The dorsal plates, in most turtles, form a median row of 5 plates; 

 the lateral plates are paired and consist of a row of 4 or 5 plates on each 

 side of the median row; the marginal plates form the margin of the cara- 

 pace and number usually 12 pairs. The median anterior marginal is 

 called the nuchal plate. The epidermal plates of the plastron are 

 usually II or 12 in number (Fig. 128). In some turtles the epidermal 

 plates are absent, the shell being covered with a leathery skin. 



The measurements used in the identification of turtles are those of 

 the carapace, and in certain cases of the plastron also, and are always 

 made in a straight line and not to follow the curve of the shell. 



Key to the Families of Testudinata 



ai Limbs not in the form of flippers; land and pond turtles, 

 bi Shell covered with horny plates. 



Ci Tail short and without dorsal tubercles; plastron large 

 and oval, 

 di Plastron with q to 1 1 horny plates; a single anterior 

 median plate; margin of carapace not flaring, but 



turned downwards (Fig. 130) i. Kinosternidce. 



62 Plastron with 12 horny plates; an anterior pair of 

 plates; margin of carapace flaring outwards (Fig. 



131) 3. TestudinidcE. 



Co Tail very long, with a dorsal row of tubercles (Fig. 129); 



plastron small, cross-shaped; snappers 2. Chdydridce. 



bo Shell not covered with horny plates, but with a leathery 



skin 4. Trioiiycliidce. 



a2 Limbs in the form of flippers; size very large; sea turtles. 



bi Shell covered with horny plates 5. Clielo)iiidce. 



h-2 Shell not covered with horny plates, but with a leathery 



skin 6. Dermochelidce. 



Family i. Kinosternidae. — Mud and musk turtles. Rather 

 small, aquatic turtles with an elongate body; carapace elliptical, with 23 

 marginal plates, usually smooth, without a flaring edge (Fig. 130); 



