1869.] DR. J. E. GRAY ON THE TORTOISES. 211 
The genera of the latter group may be arranged according to the 
number of the sternal callosities, thus :— 
I. Sternal callosities two, lateral: Aspilus, Rafetus, Dogania. 
II. Sternal callosities four, lateral and anal: Tvionyx, Potamochelys, 
Pelochelys, Chitra, Tetrathyra, Platypeltis. 
III. Sternal callosities six, lateral, anal, arid pectoral; the latter 
transverse and developed late : Landemania. 
IV. Sternal callosities seven: Heptathyra. 
V. Sternal callosities nine (or ten): Emyda. 
VI. Sternal callosities fifteen, and often some subsidiary ones: Cy- 
clanosteus and Baikiea. 
The development of the pittings on the surface of the odd bone in 
front of the bony dorsal disk affords good generic characters. 
1. It is smooth to a comparatively later period, even after the 
sternal callosities are developed in Aspilus. 
2. It is pitted in the centre in very young, and gradually becomes 
more covered with pits in young specimens in Trionyax, Potamochelys, 
and Tyrse. 
The Mud-Turtles with a depressed thin skull and very short face 
have a broad flat palate and scarcely any indication of a central 
groove in front of the internal nostrils, and only a slight depression, 
if any, behind them; whereas in the more or less oblong, thick, 
solid skulls the palate is more or less concave, and almost always 
has a central groove in front and two more or less deep concavities 
behind the internal nostrils. The extent and form of the depressions 
afford very good generic characters. 
The skulls of the Trionychide may be arranged in sections thus :— 
1. The central groove in front of the internal nostrils narrow and— 
a, Deep: Landemania, Sarbieria, Potamochelys, ? Platypeltis, 
Callinia, ° Emyda. 
6. Very shallow: Aspilus. 
2. Centra: groove in front of the internal nostrils short, triangular, 
narrow in front: Cyclanosteus, Baikiea, Tetrathyra. 
3. Central groove in front of the internal nostrils wide and shallow, 
but well marked: Trionyx, Rafetus, Dogania, Tyrse. 
The form of and the extent of the development on the edge of 
the jaws afford excellent characters, and show the differences in 
the habits of these animals. In the Mud-Turtles with depressed, 
thin, light skull, and short face, the alveolar edges of the jaws 
are thin and linear. In those which have a more or less high, 
solid, strong skull, the edge of the lower jaw and the surface of 
the upper one that meets it offer several variations. The genera 
Se be arranged by the different forms of the alveolar surface, 
thus :— 
1. The edge of the lower jaw flattened and broad in front and 
on the sides: Trionya, Aspilus, Rafetus, Batkiea. 
