INTRODUCTION. 5 



recently obtained by parties from Princeton College. Althougli these 

 remains show few, if any, characters of the Dinoccrafa not better 

 represented in the larger collection of the Yale Museum, full references 

 to the more important specimens, in most cases with illustrations, are 

 given in the present memoir, especially in the Synopsis at the end 

 of the volume. 



The Dinocerafa have hitherto been found in a well marked geological 

 horizon of the middle Eocene. The relations of tliis horizon to other 

 deposits of Tertiary age are important, and cannot readily be understood 

 without having in mind the principal changes that took place in the 

 geology of the Rocky Mountain region dui-ing this period. These 

 changes and their results may be briefly stated as follows : 



The Tertiary of Western America comprises the most extensive 

 series of deposits of this age known to geologists, and important breaks 

 in both the rocks and the fossils separate it into three well-marked 

 divisions. These natural divisions are not the exact equivalents of the 

 Eocene, Miocene, and Pliocene of Europe, although usually so considered, 

 and known by the same names ; but, in general, the fauna of each a})pears 

 to be older thaii that of its corresponding representative in the other 

 hemisphere ; an important fact, but little recognized. This partial 

 resemblance of our extinct faunas to others in regions widely separated, 

 where the formations are doubtless somewhat ditlerent in geological age, is 

 precisely what we might expect, if, as was probable, the main migrations 

 took place from this continent. It is better at once to recognize this 

 general principle, rather than attempt to bring into exact parallelism 

 formations that were not contemporaneous. 



The fresh-water Eocene deposits of our Western Territories, which 

 are in the same region at least two miles in vertical thickness, may be 

 separated into three distinct subdivisions. The lowest of these, resting 

 unconformabl y pn the Cretaceous, has been termed the Vermilion Creek, 

 or Wasatch, group. It cojitains a well-marked mammalian fauna, the 



