80 DR. J. E. GRAY ON THE TRIONYCHIDE. [Feb. 23, 
habits, quite as important as the form of the sternum and the flexi- 
bility or inflexibility of the edge of the dorsal disks ; but so little is 
known of the habits and manners of these animals, that we have no 
materials to work from. 
A. The skull solid, convex, subtrigonal; nose moderate ; eyes la- 
teral; the palate concave, with raised alveolar edges, and a deep ob- 
long concavity enclosing and extending behind the internal nostrils. 
Emyda. Cyclanosteus. Trionyx. Tyrse. Dogania. Rafetus. 
B. Skull oblong, thin, and light ; nose very short ; eyes anterior ; 
forehead flat, often elongate ; palate flat, with scarcely raised alveo- 
lar edges, and only a very slight depression before and behind the 
internal nostrils. 
Chitra. Heptathyra. Pelochelys. 
There is a very considerable difference in the form of the grooves 
in the palate, and in the position of the internal nostrils. 
In Trionyx gangeticus the groove is very broad, equally open, and 
of an equal width, with the circular internal nostril behind on a level 
with the front of the zygomatic arch. 
In Potamochelys stellatus the groove is very narrow in front, partly 
arched over on the sides by the inner edge of the large alveolar mar- 
gin, with the oblong internal nostrils very close together, in a line 
with the middle of the upper lip. 
In Rafetus euphraticus the palate-groove is intermediate in form 
between that of Trionyx and Potamochelys, being broad, deep, en- 
tirely open, but rather narrower in front, with the large circular in- 
ternal nostrils rather in front of the zygomatic arches. 
In Cyclanosteus senegalensis the palate is somewhat like that of 
Potamochelys; but it is not so much contracted in front, and the 
oblong internal nostrils are larger, broader, and nearly in the same 
situation. 
In Tyrse nilotica the palate is regularly concave in front of the 
internal nostrils, which are in a deep pit, just before the front of the 
zygomatic arch, and this concavity is separated by a longitudinal 
ridge between the nostrils. 
There is an apparent anomaly in the development of the sternal 
callosities, which can only be solved by the conjecture that (as 
Professor Agassiz declares to be the case) there are several species 
which have very much the same external appearance. In more than 
one of the species under examination, the sternal callosities are well 
developed in some specimens, and scarcely visible in others of the 
same species and, sometimes, even of a larger size. 
I. The typical or naked-legged Mud-Tortoises have the sternum 
narrow behind, without any valves over the hind legs; only 
two or four sternal callosities, and margin of the disk flexible, 
without any internal bones. GYMNOPUS. 
Aspidonectes, Wagler, Syst. 1830. 
Trionyx, Gray, Syn. ; Griffith ; Cuvier, 1831. 
Amyda, Bonap. Tab. Anat. 19, 1836. 
Gymnopus, Dum. et Bib. Erp. Gén. ii. 479, 1835. 
