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Show 210 DR. J. E. GRAY ON THE TORTOISES. [Mar. 1 1, 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 collection has convinced m e 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 in it 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 process 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 m e 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 m y knowledge of the structure and development 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. |