ON BRITISH FOSSIL REPTILES. 165 



Thus the first median scute is in the form of an ancient shield ; its posterior 

 apex impressing and crossing the anterior apex of the entosternum. The pos- 

 terior transverse boundary of the succeeding pair of sternal scutes crosses the 

 plastron 4^ inches from its anterior margin ; that of the third pair of scutes 

 crosses at 7| inches from the anterior border, and between the two transverse 

 sutures ; that of the fourth pair at 10 inches distance from the anterior mar- 

 gin, and about 1^ inch behind the second transverse suture ; passing straight 

 across the plastron between the posterior concave margins of the lateral walls. 

 The posterior boundary of the fifth pair of scutes inclines obliquely back- 

 wards from the median line, as usual ; it is 3 inches behind the preceding 

 transverse impression. 



It is in the interspace of these impressions that traces of the transverse su- 

 ture between the hyposternals and xiphisternals are obvious, about 4 inches 

 from the posterior extremity of the plastron. If these traces were not so ob- 

 vious, it might be supposed that the xiphisternals were of unusual length, 

 entering into the formation of the lateral wall, and extending backwards from 

 the second transverse suture to the end of the plastron ; but this disproportion 

 would be hardly less anomalous than the existence of the additional pair of 

 bones intercalated between the hyo- and hyposternals which this present fossil 

 evidently displays. 



In most of the existing large Emydes and Platemydes, the median transverse 

 suture traverses the plastron a little behind the third pair of scutes, or across 

 the fourth pair ; so that the second transverse suture in the fossil has the ana- 

 logous position, and accordingly has most right to be regarded as the normal 

 boundary between the hyo- and hyposternals. One of the most distinctive 

 characters of the present extinct Platemys is, therefore, the division of each 

 hyosternal bone into two, the sternum consisting of eleven instead of nine 

 pieces ; if the very interesting anomaly which it displays be not an accidental 

 or individual variety. 



The chief difference in regard to the sternal scutes, is the addition of two 

 small ones anteriorly, one on each side of the median anterior pair in the 

 fossil. 



The obtuse ridge which forms the angle between the carapace and plas- 

 tron is preserved in the fossil. 



Tretosternon punctatum, nob. — In the rich collection of fossil remains be- 

 longing to Sir P. Egerton, there is the posterior part of the carapace of a fine 

 species of freshwater tortoise, which, by its broad and extremely flattened 

 form and sculptured surface, is evidently closely allied to the genus Trionyx, 

 but which, from the impressions of distinct horny scutes, is essentially related 

 to the Emydian family, and is nearly allied to the genus Platemys, D. & B. 

 (^Hydraspis, Bell.) : this portion of carapace contains the fifth to the twelfth 

 vertebral plates inclusive, and the five posterior pairs of expanded vertebral 

 ribs. The external surface of both elements of the carapace is closely pitted 

 with minute irregular impressions, smaller than a pin's head, and along their 

 sutural margins for the extent of two or three lines by straight and parallel 

 linear impressions, at right angles or nearly so to those margins : the pin- 

 head impressions are sparing or absent at these striated margins. 



The breadth of the carapace, across the fourth pair of ribs, is 13|^ inches : 

 the length of the moiety of the carapace here preserved is 9 inches : the en- 

 tire length would be, probably, 17 inches. The flattened ribs gradually ex- 

 pand towards their distal extremity. 



The close resemblance which this species makes to the Trionyces, in the 

 sculpturing of the external surface of the carapace, is very striking ; but the 

 impressions of the horny scutes, and the non-continuation of a narrow tooth- 



