210 DR. J. KF. GRAY ON THE TORTOISES. { Mar. 11, 
mens as well as old of the two species; and the examination of the 
jaws of the young specimens of other species in the Museum collec- 
tion has convinced me that very little, if any, change of form occurs 
in the alveolar surface of the soft Mud-Turtles of the same species 
from youth to old age, and that the various forms of the alveolar 
surface afford excellent characters for the distinction of the species 
and genera of the group, and are also in conformity with their habits 
and food. Extending the examination to the mouth of old and 
young specimens of Tortoises of other families, I find that these 
characters are equally permanent in them. 
When the above-mentioned essay was prepared, as we had only a 
limited number of skeletons, I was obliged to leave init a number of 
species doubtful as to the genera to which they ought to be referred. 
Having discovered that the characters afforded by the alveolar pro- 
cess were the same in the young specimens as in the older ones, I 
was induced to examine the mouths of all the young specimens which 
we had in spirit in the British Musenm; and finding that, by very 
careful preparation and manipulation, I could open the mouths of the 
stuffed specimens in the same collection without in the least degree 
injuring them, I have examined the mouth and alveolar surfaces in 
all of them, and thus satisfied myself of the permanence of the 
characters that these afford, and have been able to determine with 
certainty the systematic position of some species, which was before 
doubtful. 
This reexamination has also shown me the permanence and the 
importance, as a specific character, of the manner in which the odd 
bone in front of the bony dorsal disk is developed, and whether it 
is in an early or late stage of its development that it becomes pitted 
like the rest of the bones of the disk, and when it becomes united to 
the front bone of it. 
These additions to my knowledge of the structure and develop- 
ment of the animals induce me to propose the following amended 
arrangement of them. A natural arrangement of the genera can 
only be prepared by taking account of all the changes of the animal 
during growth, and deriving the characters from it in its perfect 
state. The young specimens are required in order to know the 
coloration of the species, the adult to know the perfect development 
of the sternal callosities, and those of intermediate ages to give the 
manner the odd bone in front of the dorsal disk is developed, and 
when and how it becomes fitted like the other bones of the back- 
shield; but this has rendered it impossible to interpolate in their 
proper place in the system those specimens in the Museum which 
are only in a young or imperfect state of development, not showing 
the sternal callosities or the true form of the dorsal disk. 
The Mud-Turtles with depressed head and thin depressed skull 
have a very short face, and the eyes only a very short distance from 
the end of the nose; all the genera which have a higher, strong, 
hard skull have a short face and a convex rounded forehead, except 
Tyrse, which has an elongate conical face, and the eyes considerably 
further back. 
