236 VERTEBRATE ANIMALS OF THE UNITED STATES 



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. 



a b 



Fig. 128. — Carapace (a) and plastron (b) of Chrysejnys marginata, to show the plates: 

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

 M, marginal; N, nuchal; P, pectoral; Pr, preanal (from Hurler). 



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 9 to 11 horny plates; margin of 

 carapace not flaring, but turned downwards 



(Fig. 130) I. Kinosternidce. 



do Plastron with 12 horny plates; margin of carapace 



flaring outwards (Fig. 131) 3. Tcstudinidce. 



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



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



