CHELONIA. 77 



Platemys Bowerbankii (?). Plate 40, figs. 1, 2. 



The evidence of species of Chelonia of the Fresh-water or Marsh-dwelHng family, 

 Pahdinosa, has hitherto been derived only from such parts of the skeleton of the 

 trunk as have been described, figured, and referred to the genera Platemys and Emys, 

 in the foregoing pages 62-76. 



Since those pages were sent to press, Mr. Bowerbank has been so fortunate as to 

 obtain from the Eocene clay at Sheppy the portion of fossil skull, of which two views 

 are given of the natural size in Plate 40, figs. 1,2. If these figures, and especially 

 the side view, fig. 1, be compared with the corresponding view of the skulls, PI. 11, 

 fig. 1; PI. 12, fig. 1; PI. MA, fig. 1, or PI. 25, fig. 1, a marked difference 

 will be discerned in the form and proportion of the orbit, which is smaller and more 

 nearly circular in fig. 1, PL 40. 



But the bony chamber for the eyeball forms one of the characters by which the 

 skull is distinguished in the marine and fresh-water families of the order Chelonia. 

 The orbit, for example, is always much larger in proportion to the entire skull in the 

 marine species, and commonly of the oval form, which is preserved in the beautiful 

 fossil skull of the Chelone cuneiceps, PI. 11, fig. 1 ; or with the upper and outer part 

 even more produced and angular than is there represented. In the families Fluvialia 

 (Trionyx) and Paludinosa (Emydians), the orbit is not merely much smaller in propor- 

 tion to the skull, it is circular, or nearly so, and not produced at the upper and outer 

 angle. By this character, we are led to refer the fossil skull under description to 

 the fresh-water division of the Chelonian order. 



Our choice between the Fluviatile or Paludinose families of that division is guided 

 by the formation of the border of the orbit, and by the proportionate length and the 

 form of the face or muzzle in advance of it. 



In the species of Eiiii/s {Podocnemi/s 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,' tom. v, pt. ii, 

 1825 ; 2C in the figure of the fossil, fig. 1, Pi. 40), 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, tom. 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 

 {ff in Cuviei''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 PI. 11, fig. 1 ; PI. \7 J, fig. 1, of the present work. 

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



M 



