ON BRITISH FOSSIL REPTILES. 169 



fossil remains, clearly referable to the marine genus Chelone, have hitherto 

 been found, is the Portland sandstone. 



Prof. Buckland possesses portions of the carapace and a beautiful specimen 

 of the skull of a Chelonian from the Portland sandstone, which, in the large 

 size of the orbits, the breadth but otherwise small size of the external nostril, 

 the extent of the osseous plate covering the temporal fossae, and formed prin- 

 cipally by the expanded posterior frontal and parietal bones, presents une- 

 quivocal characters of a marine species. It differs, however, from all other 

 known recent and most extinct Turtles, in having the cranium more depressed, 

 the nasal bones divided by a distinct transverse suture from the pre-frontals, 

 and by some otlier minor differences, in which an affinity to the Platemydian 

 family may be traced. The length of this cranium is 4? inches 4 lines, its 

 greatest breadth 4 inches. The chief modification of external form is the deep 

 emargination of the lower border of the cranium, between the malar and tym- 

 panic bones ; a character by which the present species approaches Emys and 

 Testudo. The Chelone longiceps of the Eocene tertiary formations makes a 

 similar but less marked approach to Emys ; and the present Turtle also re- 

 sembles Chel. longiceps* in the form of the mastoid bone, which, instead of 

 forming a thick convexity behind the wide tympanic cavity, forms a smooth 

 and slightly concave, moderately broad, semicircular plate of bone. The 

 muzzle is, however, as short in the Turtle of the Portland stone as in ordinary 

 species of Chelone ; the distance, for example, from the anterior part of the 

 orbit to the end of the muzzle, is only 11 lines. The median frontal sends a 

 narrow pointed process forwards between the pre-frontals, as far as the suture 

 which divides them from the nasal bones. The breadth of the interorbital 

 space is relatively less than in recent Turtles, or than in the Chelone longi- 

 ceps : it measures 8 lines. 



The median frontal enters into the formation of the upper part of the or- 

 bit, in a greater proportion than in Chel. mydas : in the Chel. imbricata it is 

 excluded by the imion of the post-frontal and pre-frontal bones. The outer 

 surface of the skull is rather undulated, marked with fine strite and punctures, 

 but not rugous, as in Chel. breviceps\; the nasal bones are convex, and 

 impressed with larger pits. The upper boundary of the nasal aperture is 

 straight, the lateral ones curve to a point below : the breadth of this aperture 

 is 7 lines; that of the orbit is 13 lines. 



The nasal process of the superior maxillary is characterised by a slightly 

 raised rough portion. 



The lower jaw closely conforms to the ordinary Thalassian type ; the sym- 

 physis is convex, oblique, and as short as usual ; there is no approximation 

 to the peculiar condition of this part in the Harwich Turtle ( Chel. plani- 

 me?itu)nX). 



The suture between the supra-angular and dentary piece does not make 

 so long and sharp an angle forwards, as in the Chelones mydas and Caretta ; 

 the coronoid process is rather higher, and the dentary piece sends out a ridge 

 which seems to have bounded the insertion of the temporal muscle below. 



Thus the present cranium offers ample proof of its specific distinction from 

 that of any previously described Chelone ; and, while it has all the essential 

 characters of that marine genus, exhibits some points of resemblance to the 

 Emydians, as in the minor breadth of the interorbital space, the deep con- 

 cavity of the lower border of the skull behind the orbit, and in the form of 

 the mastoid bone. The separate nasal bone, which is the most interesting 



* See Proceedings of the Geological Society, December 1st, 1841. 

 t Ibid. X Ibid. 



