23 



Family I. CHERSITES. Dumeril ct Bihron. 



CHARACTERS. 



1. The body is short, oval, convex, and protected above by a shell or carapace, 



covered \vith plates, of which there are thirteen dorsal, never imbricated, 

 and from twenty-three to twenty-five marginal. Under this carapace the 

 animal can retract the head, neck, extremities and tail. 



2. There is a bony sternum or plastron below, which is most commonly covered 



with twelve plates, sometimes with eleven only. 



3. In some the anterior part of the sternum is movable; (Pyxis, Bell;) in 

 others the posterior; (Chcrsus, Wagler;) and in others again both anterior 

 and posterior sections are movable; (Cistuda.) 



4. The head in all is small, short, and quadrilateral. 



5. The nostrils are anterior, closely approximated, and just above the median 



border of the horny covering of the upper jaw. 



6. The eyes are lateral, and are always nearer the snout than the occiput. 



They are small, with tlie lids opening obliquely, so that the anterior canthus 

 is on a plane with the nostrils, while the posterior is much more elevated. 

 The inferior lid is most movable. 



7. The membrane of the tympanum is always apparent, circular and large. 



