EOCENE. 31 



Dinosauria are a Mesozic type, not known elsewhere from the Tertiary; (2) 

 because Mammalia (should they be found in the future in the Fort Union) 

 are not equal as evidence of Tertiary age, since they have been also found 

 in Jurassic and Triassic beds. 



The Eocene fauna is so varied, especially in Europe, that it is neces- 

 sary to compare the divisions separately, as in the case of the Cretaceous. 

 Thus, the fauna of the Suessonian is quite as distinct from that of the Cal- 

 caire Grossier and Gypse (Parisian and Tongrian) in France as are those of 

 the Wasatch and Bridger epochs in North America. 



I have already identified the Wasatch with the Suessonian or Orthro- 

 cene, on account of the community of the following genera in the two 

 continents: Coryphodon, Hyracotherium, Amhly clonus, Clastes, and a form of 

 birds close to Gastornis. I can now add Phenacodus, Orotherium (Cope), 

 and very probably Hyopsodus, Adapis, Opisthotomus, and Prototomus. But, 

 as above mentioned, in the lower beds of the Suessonian in France occur 

 genera which are, so far as yet known, wanting in the Wasatch of America, 

 but present in the beds of the Laramie. 



The parallelism of the American Wasatch with the Upper Suessonian 

 of France is the second identification which may be regarded as provision- 

 ally established. The only important discordant element at present known 

 is the Tceniodonta of the Wasatch, which have not so far been found in 

 Europe. 



Above the Suessonian, a divergence in the characters of the European 

 and North American faunae commences, and continues to be marked through- 

 out the remainder of Tertiary time. So far as the Mammalia are concerned, 

 the diversity between the continents was greater during the periods of the 

 Upper Eocene and Miocene than at the present era. During these periods, 

 a limited number of genera, common to the two continents, was associated 

 with numerous genera in the one which did not exist in the other. As a 

 consequence, our paleontological means of identification of the horizons 

 are limited to a restricted list, and the task of applying a uniform nomencla- 

 ture is, under the circumstances, diflScult. Another difficulty in the way of 

 determining the place of the American beds in the Eui-opean scale consists 

 in the fact that the physical history of the two continents dui'ing the Ter- 



