GEOLOGICAL DISTRIBUTION. 41 
and Portlandian; likewise, in the Wealden, of the Lower Cretaceous deposits of 
approximately the age of the Trinity and Potomac. In the Portlandian is found a 
species of Pleurosternon. In the Wealden occurs the genus Tretosternon, which 
may have given origin to the Dermatemydidz. 
In the Gault, at the base of the Upper Cretaceous of England, have been found 
remains of a large turtle which is called by Lydekker Chelone jessont (Quart. Jour. 
Geol. Soc., xLv, 1889, p. 231). While it is possible that this species belonged to the 
Chelonidz, it is improbable that it was congeneric with our living green-turtle. It 
would be more proper to call it Cimochelys jessont. 
The Cambridge Greensand, of the Cenomanian, near the time of the American 
Dakota or Benton, furnishes a number of turtles. “These are represented by skulls 
alone and are placed in the genus Rhinochelys. heir affinities are uncertain, but 
they are neither Pleurodira nor Trionychoidea. The genus seems not to be repre- 
sented in America. 
In the Lower Chalk, Turonian, of Kent, England, are found remains which are 
assigned by Lydekker (Cat. Chelonians, 1889, p. 34) to Chelone benstedt. For the 
reason noted above, this species ought to be called Cimochelys benstedi, as it was 
named by Owen. The Turonian was deposited approximately at the time of our 
Niobrara beds. 
In the Upper Cretaceous of England and the continent is found a huge marine 
turtle, Allopleuron hoffman; its costals are greatly reduced. In Italy has been 
found another large sea-turtle, Protos phargis, reminding us of Protostega and 
Archelon, of the American Upper Cretaceous. 
In the Upper Cretaceous of Provence, France, has been discovered the remark- 
able turtle, Polysternon. Besides mesoplastra it possesses a pair of bones between 
the hypoplastra and the xiphiplastra, and the pelvic bones are sutured to the xiphi- 
plastra. It 1s regarded as a pleurodire; and it appeared about the time that this 
group presented itself in America. 
During the Tertiary period the history of European turtles appears to have run 
on much as it did in America, altho in each country the history had its peculiar 
features. Osborn (Ann. N. Y. Acad. Sci., x11, 1899, p- 7) finds that America and 
Europe had a similar mammalian fauna throughout the Lower Eocene, but that 
there followed a Middle and Upper Eocene interval of faunal separation of the two 
countries; while again there was an approach, from the beginning of the Oligocene 
onward until the late Pleistocene. With respect to the turtles it may be said that 
the two regions were farther apart at the beginning of the Eocene, converged to 
perhaps the middle of the Pliocene, then diverged. 
In the London Clay of England is found Eosphargis, the earliest representative 
of the Athecae. In Belgium at the same time there existed Pseudotrionyx, a relative 
of the living Carettochelys. In England there were yet two species of Pleurodires. 
So far as known, the group was not represented at the time in America. In Lurope 
there were no Amphichelydia, no Dermatemydidz, no known species of Plasto- 
menide, and no Trionychide. ‘The species of Argillochelys may belong to the 
Cheloniide. Species referred by Lydekker to Chrysemys represent the Wasatch 
Emydide, while Homopus comptont represents Hadrianus. 
The Middle Eocene of Europe witnest a great multiplication in the number of 
Trionychide. Species referred to the yet living Asiatic genus Ocadia, of the 
Emydide, resembled the Bridger genus Echmatemys. The Bridger epoch differs 
from the European Eocene in its numerous species of Baénide. 
During the Oligocene, Europe possest numerous Trionychide. Doubtless in 
America, too, they existed in numbers, but they have mostly escaped preservation. 
