CHELONIA. 9 



Mr. Parkinson describes the plastron of a Sheppey Chelonite,* in which the 

 hyosternal and hyposternal pieces are not united, but leave a vacancy in the middle, 

 which he conjectured may have been filled up by membrane. This specimen must 

 have belonged to a specimen at least four inches in length, exclusive of the head 

 and neck. But Cuvier supposes that it may, nevertheless, have belonged to an 

 Emys ; and that the vacancy of the bony sternum merely indicated the nonage of the 

 individual. t 



The grounds on which Cuvier refers to the genus Emys, the imperfect and dislocated 

 carapace and plastron of M. Bourdet's Sheppey Chelonite,:}; are not detailed ; but it is 

 evident that the hyposternals in that specimen are in contact at the posterior moiety 

 of their median margins only ; and that the margins recede anteriorly, leaving a 

 median interspace ; which, as the plastron is nearly a foot in length, can hardly be 

 attributed to the immature state of the individual. And if, as Cuvier supposes, this 

 specimen belongs to the same species as those in the collections of Messrs. Crowe and 

 Parkinson, the same objection to their belonging to a fresh-water tortoise holds good^ 

 as to the one figured by M. Bourdet. 



The question of the reference of these Eocene fossils to the fresh- or sea-water 

 families of the Chelonian order, seems to me to admit of the safest determination by 

 examining the crania of the Sheppey Chelonites ; since the differences in the extent - 

 to which the temporal fossse are protected by bone, and in the proportions in which 

 the bones enter into the formation of that covering, are strongly marked in the genera 

 Emys and Chelone. 



But here Cuvier appears to have been unusually biassed in favour of the Emydian 

 nature of the Sheppey fossils ; for in reference to the cranium, figured by Mr. Parkinson, 

 the affinities of which to the turtle's skull will be presently pointed out, Cuvier observes : 

 " elle est probablement aussi d'une Emyde, bien qu'elle participe des caractcres de 

 Tortues de Mer, par la manicre dont le parietal recouvre sa tempe ; mais nous avons 

 vu que VEmys expansa differe tres peu de Tortues de Mer a cet egard, et la partie 

 anterieure de la tete fossile ressemble d'avantage a celle d'une Emyde qu' a celle d'une 

 Chelonee, surtout par le peu de largeur de I'intervalle des yeux."^ 



Now the most striking difference between the temporal bony vault of the Emys 

 expansa and that of any known species of Chelone, is seen in the diminutive size of the 

 post-frontals in this exceptional case among the Emydes, as contrasted with their large 

 size and actual extension over the temporal fossae in the Chelones : — and this difference 

 is accompanied by a proportional diminution in the breadth of the parietals in the 

 true marine turtles. 



* Organic Remains, vol. iii, p. 268, pi. xviii, fig. 2. 

 f Ossemens Fossiles, torn, v, pt. ii, p. 235. 

 X Tom. cit., pi. XV, figs. 14-15. 

 § Tom. cit., p. 235. 



