ON BRITISH FOSSIL REPTILES. 173 



bits ; the anterior extremities of the median frontals, instead of converging to 

 a point, are extended forwards, between the anterior frontals, in a broader 

 proportion than in the Portland Turtle, and are obliquely truncated : it is only 

 in the genus Chelys among existing Chelonians, that the anterior frontals are 

 thus separated from each other ; but in the Chelys the intervening extremities 

 of the median frontals are continued to the upper border of the external 

 nostril. In the present fossil cranium the oblique extremities of the anterior 

 frontals are arrested at the distance of four lines from the nasal aperture, which 

 is bounded above by two distinct nasal bones ; these bones are joined by su- 

 ture to the median frontals, to the anterior frontals, and to the superior max- 

 illaries ; the nasal processes of which extend upward, and exclude the anterior 

 frontals from the nasal boundary. The superior maxillaries are traversed 

 obliquely by a large and deep scutal impression, above which the superior 

 maxillary forms a convex prominence at the anterior part of the orbit. The 

 scutal groove which traverses the median frontals is as strongly marked ; that 

 which impresses the post-frontals is fainter. The expanded trumpet-shaped 

 portion of the tympanic bone comes nearer the upper margin of the cavity 

 than in existing Chelones. 



The palatal bones have no true palatal process. The palatal processes of 

 the intermaxillary and maxillary bones form an unusually prominent angular 

 ridge, running nearly parallel with the trenchant margin of the jaw : the bony 

 palate is not extended along the middle line beyond the intermaxillaries. The 

 pterygoid bones present moderately wide and deep external emarginations. 



In. Lin. 



Length of the cranium from the occipital tubercle . 2 3 



Breadth of the cranium above the tympanic cavities . 1 6 



Depth of the cranium at the parietal bones .... 1 



Antero-posterior diameter of the orbit 10 



Breadth of the interorbital space 8 



Chelone Benstedi, nob. 

 Emys Benstedi, Mantell. , 



Although very characteristic remains of Chelonian reptiles have been de- 

 termined by Cuvier, from the cretaceous beds of St. Peter's Mount, near 

 Maestricht, no evidence of the present order of Reptiles in British chalk 

 formations had been made public until my description of a Chelonite from 

 the lower chalk at Burham, Kent, in the museum of Sir P. Egerton, appeared 

 in the Proceedings of the Geological Society. This Chelonite consisted of 

 four marginal plates of the carapace, and a few other obscure fragments, 

 sufficient to prove that the species was not Trionyx or Testudo : and as they 

 differed in form from those of the recent species of Chelone, with which I 

 compared them, and resembled rather the posterior marginal plates of some 

 Emydians, I stated that this correspondence " rendered it probable that these 

 remains are referable to that family of Chelonia which lives in fresh water or 

 estuaries." Subsequent observation of the various interesting modifications 

 by which extinct Chelones diminish, as it were, the gap between the marine 

 and freshwater genera as they remain at the present day, has weakened the 

 impression which the character of the marginal plates of the chalk Chelo- 

 nite first made in favour of its Emydian affinities ; and the examination of the 

 beautiful Chelonite, obtained from the same quarries at Burham, and relieved 

 from the chalk matrix by Mr. Bensted, lately described and figured by Dr. 

 Mantell in the Philosophical Transactions, has demonstrated that it is not an 

 Emys but a true Chelone. 



