230 VERTEBRATE ANIMALS OF THE UNITED STATES 



noted for their ferocity and voracity; aquatic, living on tish. water- 

 birds and other animals; flesh valued for food. 



Key to the Genera of Cheiydridae 



ai Head covered with a soft skin i. Chelydra. 



a2 Head covered with symmetrical plates ". 2. Macrochelys. 



1. Chelydra Schweigger. Carapace v^ith 3 blunt keels, more or 

 less pronounced, growing less so with age ; tail very long, with 2 rows of 

 scales beneath; eyes superior in position: i species. 



C. serpentina (L.). Common snapper. Length 700 mm. or more; 

 length of carapace 300 mm.; width 260 mm.; weight 30 lbs. or more; 

 2 chin barbels; a row of large compressed tubercles on the upper surface 

 of the tail (Fig. 129); eggs spherical, 25 mm. in diameter, about 25 in 

 number: North America, east of the Rockies, from Canada to the Gulf; 

 common. 



2. Macrochelys Gray. Head very large and covered above with 

 plates; tail with small scales beneath; eyes lateral in position: carapace 

 with 3 very prominent tuberculated ridges: i species. 



M. temmincki (Holbrook). Alligator snapper. Length 1,000 mm.; 

 length of carapace 620 mm.; width 530 mm.; weight 120 lbs.; eggs t^^ 

 mm. in diameter: Gulf States; northward in the Mississippi Valley 

 into northern Missouri. 



Family 3. Testudinidae. — Pond and land turtles. Carapace 

 ovate and with flaring edges (Fig. 131) and either depressed or more or 

 less strongly convex, and covered with 5 dorsals, 4 pairs of costals and 

 25 marginals; plastron covered with 12 plates, and of large size and 

 firmly joined with the carapace by wide bridges, and in some species 

 with a movable anterior lobe enabling the animal to close its shell; 

 toes 5-4; egg elliptical: about 80 species in the United States; many 

 species are used for food. 



Key to the Genera of Testudinidae 



ai Digits spreading, not closely bound together. 



bi Plastron without hinge, immovably joined to carapace. 



Ci Alveolar surfaces of jaws broad; hind feet largest, broadly 

 webbed, 

 di Alveolar surfaces of jaws smooth; upper jaws not 

 notched in front ; carapace keeled. 

 Ci Lower jaw not spoon-shaped at tip; plates of cara- 

 pace rugose. . . . : 4. Malademys. 



e2 Lower jaw spoon-shaped at tip; plates of carapace 



smooth 5. Grdptcmys. 



