ON BRITISH FOSSIL REPTILES. 175 



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 spe- 

 cies 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 hypo- 

 sternals joined with those processes by an inward development of their in- 

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

 border of the contiguous free marginal plates. The outer contour of the 

 ninth, tenth, and eleventh plates projects in the form of a slight angle, and 

 thus differs from the same parts of Chelone mydas and Chelone obovata ; 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 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 upjier 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 

 P. Egerton. The form of the median or vertebral scutes is only to be traced 

 at the anterior part of the carapace, but their relative breadth and the out- 

 ward 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. 



The coracoid bone varies in form so as to be very characteristic of the dif- 

 ferent genera of Chelonians ; it is a triangular plate in Testudo, a more elon- 

 gated 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, fortunately preserved in the present specimen, here exhibits, 

 showing that the same modifications of the skeleton are combined in the past 

 as in the present species of Chelone ; it is 1 inch 7 lines in length, cylindrical 

 at its humeral half, and gently expanded to a breadth of 3 lines at its sternal 

 end. The proportion which this bone presents of one-fourth the length of 

 the carapace is only paralleled in the existing Chelones ; it is much shorter 

 in the Emydes. 



The hyosternal and hyposternal bones resemble rather those of the turtles 

 than of the young Emydes ; certainly no Emys, with a carapace 5 inches in 

 length, presents such forms as these bones exhibit in the present fossil; 

 several rays or pointed spines of bone are developed from the anterior half of 

 the median margin of the hyosternal piece, as in Chelone caretta ; the rest of 

 the margin contributes to form the circumference of the large central aper- 

 ture of the sternum. The hyposternal sends similar rays from the posterior 

 half of its outer margin, leaving the anterior half to join, probably the same 

 proportion of the outer margin of the hyosternal, so as to form a deep lateral 

 angular notch of the sternum. The length of the hyposternal is 1 inch 

 2 lines. The epi-, ento- and xiphisternal bones ai-e not preserved. 



From the preceding description it must be obvious, as has been already 

 observed, that the present Chelonite of the chalk can only be supposed to 

 belong to the genus Emys, on the supposition that it is a very young spe- 

 cimen of some unusually large species : but against this supposition, the 

 pointed form of the hind end of the carapace, the regularity of the size of 



