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Show 1873.] DR. J. E. GRAY ON THE GENERA OF TURTLES. 403 hinder edges of the last pair of ribs : but the re-examination of a larger set of specimens of different ages makes me doubt the importance of the characters assigned to them; for I cannot find, in a very large series of specimens, any character but such as is altered with age, and seems common to the whole group, either in the form of the skull or form and development of the bones of the disk. Though the middle-aged specimens are differently coloured, as if indicating two species, all the very young specimens are very much alike, as if they belonged to one species. It is the same with the form and development of the shell and of the dorsal and sternal disks, which are all liable to slight and apparently unimportant variations ; and evidently, from the dorsal shells we have received, the animals of which have been cooked, the green and rayed Turtles are both eaten. At the same time it should be recollected that museum zoologists labour under great disadvantages ; and if the two species or varieties could be examined alive and their skeletons compared, a character might still be found to distinguish them. Fam. 2. CAOUANIDCE. The head broad, covered with regular symmetrical shields, with three or four pairs of shields over the orbit, and two shields on each side of the occiput. The beaks large and horny, the lower one just fitting into the edge of the upper, the upper and lower beaks occupying the greater part of the lateral margin of the jaws ; the hook of the lower beak fitting into a pit in the alveolar surface of the upper one; the lower beak covering the greater part of the lower jaw, which has small scales on the hinder part of the sides. Tribe 1. CAOUANINA. The jaws strong. Costal shields five on each side, the front shield small and thin ; hinder ones broad, as broad as two marginal shields. The nuchal shield very broad, as broad as the first vertebral. The alveolar surface of the upper beak and skull beneath it smooth, with a deep pit in front for the acute point of the lower beak and lower jaw. The base of the skull of Caouana is nearly flat, with a narrow groove behind diverging at each side of the front edge of the rather prominent triangular basisphenoid bone. The tympanic cavity has a smooth naked space in front of it, which is flat, and not concave as in the Cheloniadee. In Caouana the front pair of sternal bones is narrow, and the front odd bone is lanceolate, rather broad. The inner edge of the front and hinder pairs of lateral bones has many radiating acute processes. The radiating processes on the inner side of the front pair are directed forwards, and those of the hinder pair are directed backwards. The two central bones between the hinder edges of tbe last pair of ribs are thick and keeled externally, comparatively short in the young specimens, and do not reach the hinder margin of the caudal marginal bones. The costal and vertebral shields of the young are 26* |