172 BRITISH FOSSIL REPTILES. 



In these extinct species, the Ufe-periods of which successively stretch backwards 

 in time from the oldest Tertiary to the newer Secondary Epochs, there is plain 

 evidence of a gradual breaking down of the distinctions that now trenchantly divide 

 the fresh-water from the marine species : the actual interval being then filled up by 

 several well-marked species, that have apparently perished. 



The Thalassian affinities of the Emydoid Chelones of the Eocene Period were, 

 nevertheless, in some instances well established by the structure of the shell, and by the 

 forms and proportions of the limbs, — parts, which it is important to bear in mind, are 

 more constant in their nature than the dermal ossifications on which tlie solidity or 

 otherwise of the carapace and plastron depends. And it must also be remembered, 

 that with the transitional species, there were associated good typical forms of Turtle, 

 e. g., Chelone planimentum and Chelone crassicosfata, as well as of Fresh-water 

 Tortoises ; e.g., Emys levis, Emys hicarinata, Platemp Bullockii. 



The Chdonitc from the Maidstone Green-sand, which forms the subject of the 

 present section, deviates from the typical Emydian structure in the arrest of the 

 dermal ossification requisite for the complete solidification of the plastron, and, 

 perhaps, also in the form of the third pair of marginal plates ; but, with the exception of 

 this doubtful point, the structure and form of every other element of the carapace are 

 more strictly Emydian, than in the most modified of the Eocene Chelones above cited ; 

 and the Emydian afiinity is more decisively shown in the form of the hyosternal ele- 

 ment, Pis. 47 and 48, fig. 1, hy. The departure of which from that of a mature typical 

 Emys does not bring it so near to the form of the same element in the typical Chelone, 

 as it does to that of an immature Emys, PI. 48, fig. 12. In the nature and amount 

 of departure from the Emydian type recognisable in the Protemys serrata, there is 

 plainly to be seen an arrest of the development of the plasti'on, which so far as it 

 has proceeded, has followed that type : there is no trace of a deviation from the 

 embryonal common fundamental pattern of the part towards the special modifications 

 characteristic of the genus Chelone. 



In the small Turtle from the Chalk {Chelone Benstedi) the ossification has extended 

 from the hyosternal and hyposternal centres by many diverging rays ; the inferior 

 plates of the marginal bones, PI. 42, fig. 1, 4 — 12, are feebly and subequally developed 

 throughout ; and there are other differences from the Protemys serrata of the Green- 

 sand, which no degree of immaturity in the Chalk specimens exhibiting them would 

 explain, as, e.g., the carinated neural plates. Pis. 41 and 43, «, «, and the pointed 

 pygal plate, PI. 41, fig. 1, py. 



Were a recent form of Emydian, so modified as the large species from the 

 Maidstone Green-sand, to be presented to the study of the modern Erpetologist, one 

 cannot doubt, but that it would be referred to a distinct sub-genus in the Fresh-water 

 family ; and I have accordingly characterised such, as far as the condition of the 

 Chelonite in question will permit. It is to be hoped, that future discoveries may bring 



