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Show 1 ('6 DR. J. E. GRAY ON THE TORTOISES. [Mar. 1 1, almost entirely covered with a bony case formed of the dilated zygomatic arch, as in the Sea-Turtles (Cheloniadee), and the other with those muscles only covered with skin, and protected externally by a broad band-like zygomatic arch, as in the Tortoises (Testudinidcc and Trionychidce); and I observed that the same difference in the form of the skull was to be observed in the genera of the Emydidee; but some genera, as Geoemyda and Cistudo, like several genera of Chelydidee, are even without any zygomatic arch, the temporal muscles being only covered with skin between the orbit and the tympanic bone as on the temple and crown. The families may be divided, according to the state of the temple, thus:- Temple bony. I. Feet clavate. Terrestrial II. Feet palmate. Fluviatile. A. Thorax covered with bony plates. a. Pelvis free below ; sternal shields 8 or 12 Platysternidee. b. Pelvis attached to the sternum; sternal shields 13 Podocephalidce. B. Thorax covered with skin III. Feet fin-shaped Cheloniadee. Spharyididce. The horny beak of these animals not only forms a cutting instrument for the separation of the food from the mass, but it also covers the chewing surface on the sides of the jaws, there being a more or less extended plate on the inside of the jaws for this purpose. In some the surface of the bone and the horny covering is smooth, as in Malaclemys and Chelydra. In general there are one or more ridges on the upper jaw fitting into grooves in the lower jaw. In the Tortoises and some of the more terrestrial Emydidee, the ridge and groove are simple; in the more aquatic Terrapins (as Pseudemys and Ba-tagur) they are more numerous and wider. Unfortunately, the form of the masticating surface is not to be usually seen in stuffed specimens; so that it is only known in a limited number of species. It must have great influence, or, rather, it shows that there is great variation in the habits of the animals, and ought to be studied for the natural arrangement of the groups. Indeed I can only regard the notes I am now making as the breaking of the sod, and consider that much has to be done before one can arrive at a satisfactory history of the habits and structure of these creatures, and form an arrangement of them consistent with their habits and manners and peculiarities. I. LAND-TORTOISES-TESTUDINIDiE. Skull solid. Orbit complete, lateral, large, hinder edge moderate. Zygomatic arch strong, well developed, united to the ear-bone behind, with a large cavity for the temporal muscle above. Temporal muscles covered with skin or horny plates. Temple fleshy. Testudinii/ce. Cistudinidce. Emydidee. Chelydradee. Chelydidee. Trionxi chides. |
