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 import- 

 ance 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 appa- 

 rently 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 stdl be found to distinguish them. 



Fam. 2. Caouanid^e. 

 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 aiid lower beaks occu- 

 pying 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 verte- 

 bral. 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 Caouaua 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 Cheloniadce. 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 the last pair of 

 ribs are thick and keeled externallv, 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 arc 



26* 



