148 



Suborder TESTUDINATA. 



Carapace usually very complete, formed by the expanded spinous pro- 

 cesses of the vcrtebrse, the expanded ribs, and a series of dermal marginal 

 bones. Plastron consisting of 8 to 11 bones; commonly united by suture 

 with the carapace. Both carapace and plastron, with one exception, cov- 

 ered with large, symmetrically arranged epidermal plates. Jaws covered 

 with horny sheaths ; not hidden by fleshy lips. Fourth digit never with 

 more than 3 phalanges. 



Key to the Families of Testudinata. 



Plastron narrow, cross-shaped; bridge long and narrow ; marginals 23^ 

 not including the nuchal ; tail long. Chelydridce, p. 148. 



Plastron of small to moderate size ; bridge short, but wider ; margi- 

 nals 28 ; tail short. Kinosternidce, p. 150. 



Plastron filling the openings of carapace, or nearly so ; marginal plates 

 25 ; tail short to moderate. Testvdinidce, p. 155. 



Family VIII. CHELYDRID^. 



Body broad and depressed ; the shell highest in front, serrated along 

 its posterior border. Plastron formed of 9 bones ; small and cross- 

 shaped ; the bridge narrow. Abdominal scutes separated from those of 

 plastron by a series of inframarginals. Head large, jaws strong and 

 hooked. Tail long, with one or more rows of compressed, horny tuber- 

 cles above. Eggs spherical. 



Genera two, both of which are represented in Indiana. 



Shell without additional plates between the marginals and costals. 



Chelydra, p. 148. 

 Shell with 3 or 4 extra plates between the marginals and costals. 



Macrocemys, p. 151. 



Genus CHELYDRA, Schweigg. 



(jhelydra,Scl&yf eigger, 1814, 88, 23; Agassiz, 1857, 4-, Ij 416; Boulen- 

 ger, 1889, 8i, 20. 



Carapace with three tuberculated keels, which disappear more or less 

 with advanced age. No supernumerary scutes intervening between the 

 marginals and the costals, just above the bridge. Plastron small, with 

 five pairs of scutes, the abdominals apparently displaced and covering the 

 bridge. Head large, jaws hooked. Head with the skin marked oft" into 

 somewhat symmetrical plates. Tail with two rows of large scales beneath. 



