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Show 392 DR. J. E. GRAY ON STERNOTHEERUS. [Apr. 1> April 1, 1873. Robert Hudson, Esq., F.R.S., V.P., in the Chair. The following papers were read :- 1. On the Skull of Sternotheerus. By Dr. J. E. GRAY, F.R.S. &c. [Received February 21, 1873.] In my essay "On the Genera of Chelydidee, and the characters furnished by the study of their Skulls "(P. Z.S. 1864, p. 128)1 was unable to describe the skull of the genus Sternotheerus, as I did not possess or know of the existence of any skull of the genus ; but I ventured to place it in tbe same section as the genus Pelomedusa, because its head was covered by hard, horny, symmetrical plates, and because I thought I felt the zygomatic arch well developed through the skin-a peculiarity that separates the African Chelydidee from the S.-American and Australian genera of this group of Tortoises, and which induces m e now to consider them a separate family, characterized by tbe structure of the skull, which in the African genera is most completely developed, while in the Australian and S.-American animals it is more or less rudimentary. I did not figure the skull of Pelomedusa, because it had already been well figured in the Atlas of Wagler's ' Natiirl. Syst. Amphib.' t. ii. figs. 36-44. This figure shows how very different the skull is from that of other Hydraspidee, but does not show the base of the skull, palate, and alveolar surface, which is so important as indicating the food of the animal, and presents modifications that afford good zoological characters. Since that period we have fortunately discovered a very large skull, evidently belonging to the genus Sternotheerus, which M . du Chaillu bad used (as he did the new species of Bush-buck which I described in the P. Z. S. 1871, p. 594) to stuff out the skin of a large African mammal. This skull is very much larger, nearly three times as large as any specimen of Sternotheerus which we have in the collection, and it shows that I was right in referring this genus to the same group as Pelomedusa. It has all the characters of that group more fully developed : that is to say, the skull is very much depressed ; the zygomatic arch is very strongly developed, being very broad and prominent ; the middle of the upper jaw is prominent, notched on each side ; and the lower jaw is very strong, with an acute produced central edge fitting into a deep pit in the front of the alveolar surface of the upper jaw, just within the prominent central margin. The beaks covering the jaws are horny, and conform to the shape of the bones, the middle of the upper part of the upper beak having a broad notch for the nostrils, the beak on the side of the nose going up to the upper surface of the nasal hole. |
