42 DR. J. E. GRAY ON THE TRIONYCHID^E. [Jan. 7. 



The hinder sternal callosities large, rather far apart. A well-de- 

 veloped callosity on each side of the front pair of bones, and a small 

 lunate callosity on the odd bone in front of the sternum. 



1. Heptathyra frenata. 



Aspilochelys livingstonii, Gray, P. Z. S. I860, p. 6, t. 22 (shell). 



Heptathyra frenata, Gray, P. Z. S. 1864, p. 93, figs. 13-15 

 (skull). 



Heptathyra aubryi, Gray, Suppl. Cat. Sh. llept. p. 93, fig. 30 

 (skull). 



Hub. Western and Central Africa. 



Fam. Trionychid^e, Gray, Suppl. Cat. Sh. Rept. p. 94. 



I. Triony china. Hinder lateral edge of 'the sternum narrow, exposing 

 the" hind feet ; the front pair of bones of the sternum without 

 any callosities ; the front odd bone of the dorsal disk more or 

 less united to the ribs, without any bone in the margin 

 before it. 



A. Dorsal disk with a broad transverse single or odd bone in front, 

 which is united to the rest of the disk by a straight suture in 

 the adult ; lateral and posterior pair of bones of the sternum 

 expanded, and covered with well-developed callosities on the 

 whole surface. 



In the young specimens the odd bone is more or less separate from 

 the dorsal disk, but is generally narrow and transverse. As this 

 bone becomes more developed it expands in length and breadth, and 

 becomes nearer to the anterior edge of the first rib. As it grows 

 larger, there are usually a couple of circular vacancies between the 

 odd bone and the front of the first pair of ribs ; but these circular 

 cavities diminish in size as the animal increases in age, and are entirely 

 obliterated by the development of the bones. 



Synopsis of the Genera of this Tribe. 



* The central palatine groove in front of the internal nosti'ils nar- 

 row, linear ; alveolar surface wide. 



1. Fordia. Skull depressed, broad ; palate nearly flat. Africa. 



2. Nilssonia. Skull high ; palate deeply concave. Asia. 



** Central palatine groove in front of the internal nostrils ivide, 

 shallow, nearly as wide as the front of the internal nostrils. 



f Front of the alveolar surface of the lower jaw wide, flat or 

 slightly concave, generally with a central longitudinal ridge. 



3. Trionyx. 



4. ISOLA. 



