TORTOISES. 13 



Fain II. Terrapens, (Emydidje). 



Head rather depressed, covered with a hard or soft skin : jaws 

 naked : nostril small, apical. Neck retractile into the cavity of 

 the thorax. Feet depressed, expanded : toes 5-5 or 5-4, almost 

 always webbed to the claws : claws sharp. Tail conical, shielded be- 

 neath. Tborax generally depressed, solid, with a distinct bony mar- 

 gin, covered with horny plates. Di?cal plates 13, marginal 24-26, 

 caudal always separate ; sternal shields 11 or 12, gular pair some- 

 times united. The vertebrae of the neck bent in a perpendicular 

 bow. Pelvis only united to the vertebrae. Rapid ; living in fresh- 

 water ponds. Carnivorous, only taking their food while in the 

 water. Egg oblong, white. 



Synopsis of the Genera. 



A. Head and tail moderate, sterno-costal suture simple. 



a. Sternum solid, truncated before and nicked behind, attached 

 to the thorax by a bony symphysis ; sternal plates 12; axil- 

 lary and inguinal plates moderate or none. 



Geoemyda. Head covered with a thin skin : toes 5-4, free, short. 



Emys. Head covered with a thin hard skin : toes 5-4, webbed. 



Tetraonyx. Head covered with a thin hard skin : toes 4-4, 

 webbed. 



Malaclemys. Head depressed, covered with a soft skin : toes 

 5-4, webbed. 



b. Sternum divided by a central cross suture, attached to the tho- 

 rax by a ligamentous suture ; sternal plates 12 ; axillary and 

 inguinal plates very small. 



Cistuda. Sternum rounded before. 



c. Sternum divided by two cross sutures, central lobe attached to 

 the thorax by a bony symphjsis ; sternal plates 11 or 8 ,• the 

 axillary and inguinal plates large. 



Kinosternon. 



B. Head and tail very large, sterno-costal symphysis covered with 

 one or three peculiar plates. 



d. Sternum cross-like, acute before; sternal plates 10, with a 

 broad one on each side, over the sides of the sternum. 



Chelydra. Tail crested. 



e. Sternum broad, truncated before; sternal plates 12; sterno- 

 costal suture covered with three additional plates. 



Platysternon. Tail cylindrical, shielded. 



C 



