GEOLOGICAL DISTRIBUTION. 41 



and Portlandian; likewise, in the Wealden, of the Lower Cretaceous deposits of 

 approximately the age ot the Trinity and Potomac. In the Portlandian is found a 

 species ot Pleurostemon. In the Wealden occurs the genus Tretosternon, which 

 may have given origin to the Dermatemydidae. 



In the Gault, at the hase of the Upper Cretaceous of England, have been found 

 remains of a large turtle which is called by Lydekker Chelone jessoni (Quart. Jour. 

 Geol. Soc, XLV, 1889, p. 231 ). While it is possible that this species belonged to the 

 Chelonidae, it is improbable that it was congeneric with our living green-turtle. It 

 would be more proper to call it Cimochelys jessoni. 



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 Rhmochelys. Their affinities are uncertain, but 

 they are neither Pleurodira nor Trionychoidea. The genus seems not to be repre- 

 sented in America. 



In the Lower Chalk, Turoman, ot Kent, England, are found remains which are 

 assigned by Lydekker (Cat. Chelonians, 1889, p. 34) to Chelone benstedi. For the 

 reason noted above, this species ought to be called Cimoehelys 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 hoffmant; its costals are greatly reduced. In Italy has been 

 found another large sea-turtle, Protosphargis, reminding us of Protostega and 

 Archelon, ot the American Upper Cretaceous. 



In the LIpper 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 thexiphi- 

 plastra. It is 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., xm, 1890, p. 7) finds that America and 

 Europe had a similar mammalian fauna throughout the Lower Elocene, 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. 



[n the London Clay ot England is found Eosphargis, the earliest representative 

 of the Athecae. In Belgium at the same time there existed Psendotrionyx, 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 Europe 

 there were no Amphichelydia, no Dermatemvdida?, no known species of Plasto- 

 menidae, and no Trionychidae. The species of Argillochelys may belong to the 

 Cheloniida?. Species referred by Lvdekker to Chrysemys represent the Wasatch 

 Emydida?, while Homopus comptont represents Hadrianus. 



The Middle Eocene of Europe witnest a great multiplication in the number of 

 rrionychidae. Species referred to the yet living Asiatic genus Ocadia, of the 

 Emydidae, resembled the Bridger genus Echmatemys. The Bridger epoch differs 

 from the European Eocene in its numerous species of Baenida?. 



During the Oligocene, Europe possest numerous Trionychidae. Doubtless in 

 America, too, they existed in numbers, but they have mostly escaped preservation. 



