32 FOSSIL REPTILIA OP THE LONDON CLAY. 



the symphysis so depressed or so sHghtly convex below as it is in the Bracklesham 

 Chelones. 



These also differ amongst themselves in this respect. The symphysis which I have 

 referred to the Chelone trigoniceps, is the broadest and flattest ; at its back part it shows 

 a deep and broad genio-hyoid groove ; this is reduced to a transversely oblong foramen 

 in Chelone mydas. 



The second species from Bracklesham, is indicated by the maxillary symphysis, 

 the sides of which meet at a more acute angle, and it is narrower in proportion 

 to its length, is more convex below, and more concave above, with the alveolar 

 borders a little more raised, and the middle line less raised than in Chelone 

 trigoniceps. In this respect it is intermediate between the Chelone imbricata, where the 

 upper surface of the symphysis is more concave, and the Chelone caouanna, where it is 

 flatter than in the Chelone trigoniceps. The fossil symphysis under notice, has also a 

 smooth, transverse, genio-hyoid groove at its back part. It accords so closely in form 

 with the end of the upper jaw of the Chelone longiceps, from Sheppey, that I refer it 

 provisionally to that species. 



Two other specimens of the symphysis of the lower jaw, of rather larger size, 

 appear to belong to the same species as that referred to the Chel. longiceps, by the 

 characters of the concavity of the upper surface, the convexity of the lower surface, 

 and the degree of convergence of the sides or borders of the symphysis. The larger 

 of the two shows the genio-hyoid groove, and the nearly vertical outer side of the 

 jaw, opposite the back part of the symphysis, and this shows no impression of the 

 smooth fossa receiving the insertion of the biting muscles, whereas, in the Chelone 

 trigoniceps, fig. 11, that fossa extends to the same transverse line or parallel with the 

 back part of the symphysis. 



The very rare and interesting Chelonite in Mr. Coombe's museum, was the first 

 portion of the cranium of a reptile of this order that I had seen from the Eocene 

 deposits at Bracklesham. It includes the bones forming the roof of the mouth, with 

 portions of the bony nostrils and orbits, and the tympanic pedicles. 



The extremity of the upper jaw is broken off, but the straight converging alveolar 

 borders clearly indicate the muzzle to have been pointed, as in the Chelone longiceps of 

 Sheppey ; and the muzzle being shorter, the form of the skull has more nearly 

 approached that of a right-angled triangle. The whole cranium is broader and shorter, 

 and the tympanic pedicles wider apart. The middle line of the palate developes a 

 somewhat stronger ridge ; the orbits were relatively larger and advanced near to the 

 muzzle : the malar bones are more protuberant behind the orbits, and their external 

 surface inclines inwards as it descends from behind and below the orbit, to form 

 the lower border of the zygoma, which it does not do in the Chelone longiceps. 



The upper surface of the fossil shows the palatines rising to form the vomer at 

 the middle line, and the two small subcircular vacuities (occupied by membrane in the 



