2 FOSSIL REPTILIA OF THE LONDON CLAY. 



In the species of Emys {Podocnemys expansa) which I have selected for comparison, 

 as offering upon the whole the nearest approach, which any Chelonian skull at my 

 command gives, to the unique fossil in question, the malar bone (i, in Cuvier's figure 

 of the skull of Emys expansa, pi. xi, fig. 9, of the ' Ossemens Fossiles,' torn, v, pt. ii, 

 1825 ; 26 in the figure of the fossil, fig. 1, T. XXIX), becomes much contracted 

 as it approaches the orbit, to which it contributes a small part of the posterior 

 border. In the Chelones the malar bone forms a larger proportion of the orbital 

 rim (see Cuvier, torn, cit., pi. xi, fig. 1 i), and contributes more to its under 

 than its back part, which is chiefly formed by the characteristically large postfrontal 12 

 (y in Cuvier's figs.) ; and this character was manifested in the ancient Eocene 

 turtles as well as in the modern species, as may be seen by reference to the 

 bones numbered 2G and 12, in T. I, fig. 1 ; T. XV, fig. 1, of the present work. 

 The superior maxillary bone 21 {b in Cuvier's figs.) is longer in the Emys, extends 

 further back in the orbit, and is deeper at its posterior termination, than in the 

 Chelones. In all these characters, derived from the bones entering into the formation 

 of the orbit, the fossil under comparison agrees with the Emys, and, indeed, departs 

 further from the Chelones than the Podocnemys expansa does, by the much smaller 

 proportion in which the anteriorly contracted malar bone (26) contributes to the rim 

 of the orbit. In the Trionyces, the malar bone forms a larger proportion of the border 

 of the orbit than in the Podocnemys expansa, and a fortiori, than in the fossil in question. 



The choice between the Fluviatile or Paludinose tribes of the fresh-water Chelonians, 

 in the determination of this fossil, is better guided by the form and proportions of the 

 skull anterior to the orbit. In the recent Trionyces the muzzle is more acute, and in 

 most of them more prolonged than in the Ernydians, with which the fossil skull 

 agrees in the shortness of the muzzle ; whilst it departs further than most recent 

 Ernydians from the Trionycida, in the broad truncated character of its anterior termi- 

 nation. There is also a very well-marked character of affinity to the Podocnemys 

 expansa, in the smooth and shallow canal which extends from the fore part of the 

 orbit forwards to the border of the external nostril across the upper part or nasal process 

 of the superior maxillary bone (21). This groove is very accurately represented in fig. 1 , 

 T. XXIX, in the fossil ; it is rather broader in proportion to its length in the Podocnemys 

 expansa ; but so far as it has depended upon the presence and arrangement of the 

 facial scutes, it is decisive against the fossil having appertained to any species of soft 

 turtle (Trionyx), in which such epidermal parts were entirely wanting. 



The marks of the supracranial scutes in the fossil are, as in some Ernydians, too 

 feebly and obscurely traceable to permit of a satisfactory comparison of their arrange- 

 ment. The exterior surface of the prefrontal (16), frontal (11), postfrontal (12), and 

 parietal (7) bones is subrcticulate. The substance of the bones is thick and coarsely 

 cancellous. The nasal bone is connate with the prefrontal, as in most modern Ernydians ; 

 in the proportion of this compound bone the fossil resembles more the ordinary 



