21 S Illinois State Laboratory of Natural History. 



Plastron with eleven plates, with two transverse hinges. 

 Bridge formed by wings of the abdominal and the contig- 

 uous axillary and inguinal. Gular barbels present. 



ClNOSTERNID^. 



Plastron small, cruciform, with ten, nine, eight, or fewer, plates. 

 Bridge narrow. Head very large, with gular barbels. 



Chelturid^. 



Entire shell covered with a continuous skin, cartilaginous at 

 the margins. Head slender; nostrils borne at the end of 

 a fleshy proboscis; horny coverings of jaws concealed at 

 the sides by fleshy lips Trionychid.^. 



Family EMYDID^. 



Shell bony, moderately depressed or strongly convex, cov- 

 ered with horny plates, of which there are five dorsal, eight 

 costal, one nuchal, twenty-two marginal, two caudals, twelve 

 sternals, and generally two axillaries and two inguinals. 

 Head of moderate size, covered with a smooth, soft skin, re- 

 tractile within the cavity of the shell. Jaws naked. Digits 

 5-4, generally fully webbed, rarely imperfectly webbed. 



The family includes the greater part of our species. The 

 majority are aquatic, and, though not by their structure un- 

 fitted for life on land, are rarely found far away from the* 

 water. A few are terrestrial, and in such species the webs of 

 the feet are greatly reduced in size. 



SYNOPSIS OF THE GENERA REPRESENTED IN ILLINOIS. 



1 (2). Plastron immovably united with the carapace; with no 

 transverse hinge 5. 



2(1). Articulation between the plastron and carapace carti- 

 laginous; with a transverse hinge between the pectoral 

 and abdominal plates 3. 



3 (4). Carapace hemispherical. Plastron rounded before and 



behind. Digits with rudimentary webs Cistudo. 



4 (3). Carapace elongate, convex. Plastron emargiuate be- 



hind. Digits with evident webs Emys. 



5 (6). Alveolar surfaces of the jaws with no median carina. . .9. 



6 (5). Alveolar surfaces with a median carina parallel with 



the margins 7. 



