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Show 1869.] DR. J. E. GRAY ON THE TORTOISES. 175 in the British Museum, which have hitherto been referred to the family Emydidee as defined in the ' Catalogue of Shield Reptiles in the British Museum,' published in 1855, I think they may be more conveniently divided into four very natural groups, which may be called families. These groups may be thus characterized:- I. The temporal muscle covered with skin, and generally protected by a narrow band-like zygomatic arch. 1. CISTUDINIDAE. The sternum united to the thorax by a cartilaginous lateral suture, and divided transversely into two moveable portions. 2. CHELYDRADAE. The sternum united to the thorax by a bony symphysis, covered with from 7 to 11 shields; the middle portion fixed to the thorax; the front and hinder portions often separated from it by a transverse suture and moveable. 3. EMYDIDAE. The sternum united to the thorax by a bony symphysis, solid, and covered with 12 shields. n. The temporal muscle covered with a bony hood formed by the extension of the zygomatic arch. Head very large. Sternal shields 11. 4. PLATYSTERNIDAE. Asiatic. Fam. I. CISTUDINIDAE or BOX-TORTOISES. Head moderate, covered with a hard thin skin. Eyes lateral or subsuperior; pupil annular. Temporal muscle covered with the skin and (except in Cistudo) protected by a band-like zygomatic arch. Thorax covered with horny plates. Sternum very broad, attached to the thorax by a ligamentous suture, covered at the sides by the pectoral and abdominal shields, and divided across into two parts by a suture between the pectoral and abdominal plates. Sternal shields 12; the axillary and inguinal plates very small or wanting. The mastoid bone is excavated to form a tympanic cell. I have little to add to m y monograph of the species of the family printed in the ** Proceedings' of the Society for 1863, p. 173, except that the temporal muscle of the North-American genus Cistudo is only covered with skin, and the skull is destitute of any zygomatic arch between the orbit and the tympanic bone. In this respect, as well as in the position of the suture between the sternum and the thorax, this genus differs from the Lutremys of Europe and the genera found in Asia, all of which have a well-developed zygomatic arch for the protection of the temporal muscle. The skull of Lutremys of Europe is figured by Cuvier, Bojanus, and Wagler. I am not aware that the skull of the very common Cistudo clausa has been figured or described. I have not seen any specimen of the Californian Cistudo blondinsia ; but, judging from the figure of the animal in Holbrook's ** North-American Herpeto- |
