K'O BRITISH FOSSIL REPTILES. 



marginal plates are joined with the sternum. This unmistakeable evidence of the marine 

 character of Mr. Bensted's beautiful fossil is unequivocally shown at h, in PI. 12, fig. 2, 

 of the 'Philosophical Transactions' for 1841, in which, nevertheless, the fossil is 

 referred to the genus Bmys. 



With reference to the general imperfect ossification of the carapace, the deductions 

 in favour of the marine nature of the Chalk chelonite might be invalidated by the 

 hypothesis, that it was the young of some very large species of Emys ; but the existing 

 Emydians at the immature period when they exhibit the incomplete ossification of the 

 carapace and plastron, have the marginal plates opposite the lateral processes of the 

 hyosternals and hyposternals joined with those processes by an inward development of 

 their inferior border, which is suddenly and considerably broader than the inferior 

 border of the contiguous free marginal plates. 



The outer contour of the tenth, eleventh, and twelfth plates of the Chelone 

 Benstedi, projects in the form of a slight angle, and they thus differ from the same 

 parts of Chelone mydas and Chelone obovata ; most of the others have a straight free 

 margin. The marginal plates appear as if bent upon themselves to form their outer 

 margin, at a rather acute angle, receiving the extremities of the rib in a depression 

 excavated in the concavity of the angle ; they are nearly twice as long in the direction 

 parallel with the margin of the carapace than transverse to it, and they are traversed 

 in the latter direction, along the middle of their upper surface, with the groove or 

 impression of the marginal scutes. The free edge of the upper plate of the marginal 

 pieces is slightly notched above the insertion of the rib, and they correspond with those 

 of the Chelonite, from the Burham chalk pit, in the collection of Sir Philip de M. Grey 

 Egerton, Bart., F.R.S. 



The form of the median or vertebral scutes of the perishable " tortoise-shell," 

 may be traced by their somewhat wide and moderately-deep impressions. They 

 progressively diminish in size from the second to the fifth, which is the smallest, and 

 which covered the ninth and the major part of the eighth and tenth neural plates ; 

 but their relative breadth and the outward extension of their lateral angles correspond, 

 like the characters of the more enduring parts, with the type of structure of the 

 marine turtles. The breadth of the first vertebral scute is 1 inch 8 lines, that of the 

 second scute is 2 inches, that of the fifth scute is 1 inch. 



The coracoid is a bone that varies in form so as to be very characteristic of the 

 different genera of Chelonians ; it is a triangular plate in Testudo, a more elongated 

 triangle in Chelys, a broad, bent, elongated plate in Trionyx, a narrower bent plate in 

 Emys, a long, straight, slender bone, slightly expanded and flattened at the sternal 

 end, in Chelone : now it is precisely the latter form that this bone (PI. 42, fig. 2, 52, 53), 

 fortunately preserved in the present specimen, here exhibits, showing that the same 

 modifications of the skeleton, in reference to the actions of swimming, are combined 

 in the past as in the present species of Chelone; it is 1 inch 7 lines in length. 



