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Show 1873.] DR. J. E. GRAY ON THE TRIONYCHIDcE. 39 of specimens in this state only affords a general impression of the alveolar surface. The skulls of the Mud-tortoises are so uncommon, and I was so disinclined to take the skulls out of the specimens, that I availed myself of the characters which the preserved heads afforded me : but I have now determined to extract the skulls in the most careful manner from the specimens themselves ; and this has given me a greatly increased knowledge of the species, and of the characters the skulls afford. The examination of the skulls of the different specimens has had the effect of putting together specimens that had been considered not only distinct species but distinct genera, and has shown them to be only various ages of the same species, as Dogania and Sarbieria ; and at the same time it has given more important characters for tbe separation of species and characteristics for the groups to which they belong. In all the other groups of Tortoises I have only figured the skulls that belonged to skeletons ; but in this group of Mud-tortoises the number of skulls that I had was so small, that I was induced to figure and try to identify two skulls which Prof. Oldham had given to the Museum with two Asiatic species : and I am sorry to say that now we have been able to examine other skulls, one of them has been proved to have been wrongly identified; that is to say, that which I figured for Potamochelys proves to be the skull of an Emyda. In m y former papers I had only the opportunity of examining the skulls of most of the species in the heads of the stuffed specimens, or of those preserved in spirits, and which consequently had the horny coat to the alveolar process of the jaws and palate, which are naturally very different from the bones which they cover. All the skulls described in this paper have been extracted from the specimens, and have had the horny coat of the alveolar surface removed. This explains why they differ from descriptions in former papers • and those in this paper are to be considered the most correct. There is a very great difficulty in comparing animals in spirits with prepared dry specimens. The living animals and the specimens in spirit have the bony disk of the back and sternum covered with a thick skin, which, perhaps with the exception of very old specimens, entirely hides the callosities on the surface of the back and part of the sternum, which are so prominent, and from which we take many characters in the dry preserved and stuffed specimens. The rugosity or callosities seem to cover the lateral bones of the sternum simultaneously over the whole surface, except the diverging rays by which the bones are united. In the hinder pair of sternal bones the callosities form a rounded or oblong spot near the internal side, and gradually enlarge themselves so as to cover the whole surface of the bone, leaving the diverging rays. The genera and families have various relations to each other, which I think are well exhibited in the following Table, which shows in one view the alliance of Heptathyra to Cyclanosleus, and Chitra |