44 FOSSIL REPTILIA OF THE LONDON CLAY. 



of the symphysis presents an uniform concavity ; and this part of the jaw is narrower 

 and more produced. I have not yet seen the mandible of any Emydian or land- 

 tortoise resembling the present fossil so closely as some of the marine species above 

 cited. A large species of Mmys has, however, left its remains in the same deposits at 

 Hordwell as the Trionyces next to be described. 



A retrospect of the facts above detailed, relative to the fossil Chelonians of the 

 genus Chelone, or marine family of the order, leads to conclusions of much greater 

 interest than the previous opinions respecting the Chelonites of the London clay could 

 have suggested. Whilst these fossils were supposed to have belonged to a fresh-water 

 genus, the difference between the present fauna and that of the Eocene period, in 

 reference to the Chelonian order, was not very great ; since the Mmys or Cistuda 

 Europ&a still abounds on the continent after which it is named, and lives long in our 

 own island in suitable localities. But the case assumes a very different aspect when 

 we come to the conviction that the majority of the Eocene Chelonites belong to the 

 true marine genus Chelone; and that the number of species of these extinct turtles 

 already obtained from so limited a space as the Isle of Sheppy, exceeds that of the 

 species of Chelone now known to exist throughout the globe. 



Notwithstanding the assiduous search of naturalists, and the attractions to the com- 

 mercial voyager which the shell and the flesh of the turtles offer, all the tropical seas 

 of the world have hitherto yielded no more than five* well-defined species of Chelone ; 

 and of these only two, as the Chel. mydas and Chel. caouanna, are known to frequent the 

 same locality. 



It is obvious, therefore, that the ancient ocean of the Eocene epoch was much less 

 sparingly inhabited by turtles ; and that these presented a greater variety of specific 

 modifications than are known in the seas of the warmer latitudes of the present day. 



The indications which the English eocene turtles, in conjunction with other organic 

 remains from the same formation, afford of the warmer climate of the latitude in 

 which they lived, as compared with that which prevails there in the present day, 

 accord with those which all the organic remains of the oldest tertiary deposits have 

 hitherto yielded in reference to this interesting point. 



That abundance of food must have been produced under such influences cannot, of 

 course, be doubted ; and we may infer that, to some of the extinct species, which, like 

 the Chel. longiceps and Chel. planimentum, exhibit either a form of head well adapted 

 for penetrating the soil, or with modifications that indicate an affinity to the Trionyces, 

 was assigned the task of checking the undue increase of the now extinct crocodiles 

 and gavials of the same epoch and locality, by devouring their eggs or their young ; 

 becoming probably, in return, themselves an occasional prey to the older individuals 

 of the same carnivorous Saurians. 



* Mr. Gray, for example, includes the Chelone virguta and Chelone maculosa of Dumeril and Bibron as 

 varieties of the Chelone mydas. 



