4:30 Wo rtin a n — St udies of Eocen e Ma mmalia in the 



The oldest Tertiary strata containing mammalian remains 

 which are at present known are found in the San Juan Basin 

 of New Mexico, and were named by Cope the Pnerco. These 

 beds are from five hundred to eight hundred feet in thickness, 

 and apparently lie conformably upon the Laramie, or Upper 

 Cretaceous. Within the lower fifty feet are found the remains 

 of a rich mammalian fauna, thirty-one species in all, composed 

 largely of representatives of the higher, or Eutherian, sub- 

 class. Associated with them are five species of Mesozoic types 

 closely related to forms from the older Laramie beds. 



In this same region, at a distance of from five to eight hun- 

 dred feet above the Puerco layer, occurs a second fossil-bearing 

 horizon, which I have called the Torrejon stage. From this 

 bed, forty-four species have been identified, of which five are 

 of the Mesozoic type. The facies of the fauna is very like 

 that of the Puerco, and there are many genera or their deriva- 

 tives common to the two horizons, but the species are in every 

 case different. Just as the Eutherian mammals in the Puerco 

 bed below were all of sudden introduction, so there were no 

 less than twelve genera which represent entirely new and pre- 

 viously unknown types. 



Resting conformably upon the Torrejon comes the Wasatch, 

 the beginning of what we may call the middle division of the 

 Eocene, and here again is a rich mammalian fauna. Deposits 

 of this age are found in the Big Horn, Green River, and Wind 

 River Basins of Wyoming and all have yielded many remains 

 of extinct mammals. Like that of the preceding bed, the 

 fauna of the Wasatch is made up of two elements : One which 

 includes the modified descendants of the older preexisting 

 types, and another which is entirely new to the region. This 

 latter element in the Wasatch comprises not only new genera 

 and species, but entire new orders. Thus, we meet for the 

 first time with such types as the Rodentia, Artiodactyla, 

 Perissodactyla, besides whole suborders and families included 

 in the Carnivora, Amblypoda, Primates, and Insectivora. 

 Some of these types continue through several subsequent 

 stages and then disappear, while others become the dominant 

 factors in the succeeding development and finally give rise to 

 the modern mammalian fauna. What is here said of the 

 Puerco, Torrejon, and Wasatch, is likewise true of the Wind 

 River, Bridger, and Uinta of the Eocene. New and strange 

 forms constantly appear, of which no vestige nor trace has ever 

 been found in the older beds. Nor is this sudden introduction 

 of new types confined to the Eocene, but is equally true of the 

 Oligocene, Miocene, and Pliocene beds of this country. 



The most remarkable circumstance connected with the facts 

 here enumerated is that identically the same thing happened in 



