ON BRITISH FOSSIL REPTILES. ' l7l 



is nearly twice as broad as long, and has convex lateral margins : the thir- 

 teenth vertebral plate, or the last of the marginal plates, is relatively broader 

 than in existing turtles, and has its posterior margin more feebly emargi- 

 nate. The marginal p! tes differ in the superior expanse of those attached to 

 the fifth, sixth and sevt.ith ribs. That to which the eighth rib is attached 

 corresponds with +.he tenth in th" Chel. mydas, and the eleventh projects in 

 an angular form Jie i '-^rspace between the dentiform extremity of the 



eighth rib and thi. n. elfti. t^ertebral plate. The thirteenth marginal plate sends 

 a similar process between the seventh and eighth ribs : the exterior margins 

 of all the marginal plates are straight, and the carapace is bounded by an 

 unbroken contour. The diameter of the marginal plate attached to the fifth 

 rib, parallel with the axis of the carapace, is 1 inch 6 lines ; the diameter 

 transverse to the carapace is 6 lines : 



In. Lin. 



The length of the carapace 109 



The breadth of the carapace 9 6 



A considerable proportion of the plastron of the same individual is pre- 

 served, together with part of the bones of the hinder extremities, and both afford 

 essential characters of the genus Chelone. The plastron of the Chelone obo- 

 vata resembles that of the Eocene Turtles in the greater extent of ossification, 

 and especially in the greater breadth of the xiphisternals, as compared with the 

 recent species ; but it differs from all other known species of Chelone in the 

 greater depth of the notches at the anterior part of the hyosternals and at 

 the posterior part of the hyposternals, which notches correspond with those 

 that in the more fully ossified plastrons of Emijdes give passage to the four 

 locomotive extremities. The essential condition, however, of the plastron 

 of the marine turtles is preserved, first, in the defective ossification of the 

 lateral margins of the plastron between the hyo- and hyposternals, which are 

 not co-extended and united to form a lateral wall of support to the carapace, 

 as in the Emydes ; and, secondly, in the form, and union by gomphosis, of the 

 xiphisternals with the hyposternals : what proportion of the central part of 

 the plastron continued unossified, the condition of the specimen does not allow 

 of determining. There is evidence of the concavity of the sternum along 

 the middle of the under surface, as in most Chelones. 



The hyosternal is chiefly remarkable for the sudden expansion of the ex- 

 ternal radiated process, which occasions a notch at its posterior part, at the 

 lateral unossified interspace between the hyo- and hyposternal bones, almost 

 as deep as that which is anterior to the radiated process. The hyposternal 

 bone presents likewise a similar modification of form. By this peculiarity of 

 form the present species might be known by a single detached hyosternal or 

 hyposternal bone. The tooth-like process of the hyosternal, which is im- 

 planted in the xiphisternal, is received into a notch, the inner boundary of 

 which is much deeper than the outer. 



The breadth of both the' xiphisternals, taken across the termination of this 

 notch, is 3 inches : they are separated by an angular fissure for the extent of 

 an inch at their posterior interspace, but their median dentated margins meet 

 in the rest of their extent, which is about one inch and a half. 



Tlie breadth of sternum, across the hyposternal bones, is 8 inches ; the least 

 antero-posterior extent of the conjoined hyo- and hyposternals is 4^ inches. 



In this admeasurement, as compared with the transverse extent o? the same 

 bones, the Chelone ohovata differs in a marked degree from the Chel. lougiceps 

 of Slieppey, and indeed approaches nearer to the existing species of Chelone 

 than do the Eocene Turtles. The adherence to the thalassian type is like- 

 wise well exhibited in the present fossil by the forms and proportions of the 



