INTRODUCTION 



This monograph describes a group of fossil mammals comprising the family Merycoidodon- 

 tids, commonly called the oreodonts. In this treatise I have omitted any systematic consideration 

 of the family Agriochceridx, a group related to the merycoidodonts, in view of the fact that the late 

 Mr. O. A. Peterson had nearly completed an intensive study of those forms when his untimely 

 death interrupted the work. 



No new species are herein proposed, but all of the established genera and species are rede- 

 scribed, with much more complete diagnostic data in nearly every instance than were given in the 

 original descriptions. This has led me to assign various species to genera other than those given by 

 their makers, while in other species I have indicated wherein I believe synonymy lies. 



Many type specimens are figured for the first time, and additional illustrations of most of the 

 other types are presented, so that as a consequence a considerable number of the text figures and a 

 very large proportion of the plates have not been previously published. 



The Merycoidodontidse comprise one of the largest and most characteristic groups of the Ter- 

 tiary faunse in North America. In fact, skulls of Merycoidodon are perhaps the commonest middle 

 Oligocene fossils of the Great Plains. These animals apparently were confined to the western half 

 of North America and mainly to the United States, but this distribution may be due to the lack of 

 early Tertiary sediments over so much of the eastern part of the continent. 



The appearance of a typical member of the group was that of an animal with a large head, 

 short neck, long body, short legs, and a moderately long tail. In size the animals ranged from 

 about that of a large domestic cat to that of a robust wild boar. They were herbivorous, and 

 probably in the early part of their racial history they ate leaves, young shoots, and possibly tubers 

 and roots, but this method of subsistence soon changed to that of grazing which predominated 

 throughout the major part of their evolutionary history. As the climate became more arid, various 

 genera developed hypsodont teeth, since the earlier brachyodont type of dentition could no longer 

 withstand the highly abrasive action of the harsher grasses. 



Curiously enough, we find the earliest merycoidodonts in strata of approximately equivalent 

 age in Wyoming and in Saskatchewan. These strata are of upper Eocene age, Uinta in Wyoming 

 and Swift Current in Canada. Some of the Wyoming representatives are probably slightly older 

 than the Canadian. From either or both localities as time went on they spread westward to the 

 Pacific Coast, eastward nearly to the Mississippi Valley, and southward into New Mexico, as well 

 as into southern California. 



The earliest Canadian species differs slightly from the contemporaneous Wyoming forms, but 

 the fossil material from the former area is so far very fragmentary and scanty and therefore 

 unsuitable for positive identification. It may be that this northern form represents a group of these 

 animals which lagged behind the main column that came by way of the land bridge in the Eocene 

 epoch from some Palasarctic Eurasiatic center of dispersal. 



The next form for our consideration is the successor of Protoreodon, the early genus. We 

 find in Canada and in several of the Western States the genus Merycoidodon, which, I believe, was 

 the main stem stock of the family. The question arises — were there two centers of dispersal for this 

 family, one in Canada and one in Wyoming? I consider this possible, but I do not believe it neces- 

 sary to assume it as a fact. Merycoidodon is found in Montana, Wyoming, Colorado, Nebraska, 

 and South Dakota and could easily have wandered up into the Cypress Hills of Saskatchewan. 

 Canada seems to have been unfavorable for merycoidodont evolution subsequent to the Mery- 

 coidodon stage, so far as we can now determine. 



I am convinced that this family is polyphyletic in its later evolution, and it would seem that 

 genera were developing in the western area at the same time that they were advancing in the States 

 east of the Front Range; otherwise we must assume that there were successive waves of migration 



