THE NE ARCTIC REGION 173 



two, and indeed the other orders to which the mammals 

 of this fauna have been assigned, all show considerable 

 points of resemblance to one another, first in the posses- 

 sion of live toes on both limbs, which are provided with 

 neither claws nor hoofs, but with a structure somewhat 

 intermediate between the two, and, secondly, in their ex- 

 tremely small cerebral cavity. A similar, though much 

 more incomplete Fauna has been found in certain beds of 

 a corresponding age in Europe, the Genera of their fossil 

 mammals being in most cases identical with those of the 

 Nearctic Region. 



In the next stage, the Wasatch beds, which correspond 

 in age nearly to the London clay of England, a further 

 development of the same fauna is found, with, however, 

 the commencement of certain of the modern Orders ; such, 

 for instance, as the Perissodactyla (or Odd-toed Ungulates), 

 the Rodents, the Insectivores, and the Lemurs. Here, too, 

 so far as the scanty remains found in Europe allow us to 

 form a comparison, there is a close similarity between the 

 faunas of the two Regions. 



In the succeeding "Bridger beds" of Mid-Eocene age 

 is found the earliest evidence of the still surviving genus 

 Didelphys (the Opossum). Here also marine Mammals 

 and Bats appear for the first time. But comparison of 

 these remains with European forms is even more difficult 

 than in the last case, owing to the scarcity of such fossils 

 in beds of the same age in Europe. 



In the Uintah beds of the Upper Eocene we first begin 

 to find very distinct traces of differentiation between the 

 European and the North American faunas, although a 

 good many of the Genera met with are still common to 

 the two Regions. 



