224 the age of mammals 



Upper Oligocene of the Western Plains and of the Mountain 



Region of Oregon 



The Upper Oligocene of America broadly corresponds to the close of 

 the Stampian and the Aquitanian stages of Europe. In both countries it 

 is characterized negatively by the absence of the hysenodonts, the last of 

 the archaic Mammalia, as well as of the amynodonts or amphi})ious rhi- 

 noceroses. Especially characteristic are various evolution stages of the pair- 

 horned rhinoceroses (Diceratherium) , which are now armed with a trans- 

 versely placed pair of horns on the ends of the nasal bones. Another 

 common form which makes its first appearance in both the New and Old 

 Worlds at this time is the primitive beaver or castorid (Steneofiber) . 



It is difficult, however, to draw close time parallels between Europe 

 and America because of new and plainly evident faunal divergence. Of 

 the six families of American artiodactyls only the anthracotheres are rep- 

 resented in Europe, and that by very different forms from those in America. 

 Of the four families of American perissodactyls both the hyracodonts and 

 the horses are absent in Europe, although the rhinoceroses and tapirs are 

 represented by somewhat similar evolution stages. Of the ancylopods, or 

 chalicotheres, the Moropus of Oregon still awaits close comparison with 

 the Macrotherium of France; the wrist, or carpus, of Moropus is the more 

 primitive. 



The Upper Oligocene of western America is clearly divided into Early, 

 Middle, and Later, or first, second, and third faunal phases, the former being 

 seen in the upper levels of the White River group of Dakota, which contain 

 a continuation of the Plains fauna, while the second, as displayed in the 

 John Day Valley of Oregon, gives us a renewed glimpse of the mountain 

 fauna and corresponds most closely Avith the true Aquitanian of France. 

 The third is again observed on the Great Plains of Dakota, is slightly sub- 

 sequent to St. Gerand-le-Puy in age, and is by many regarded as the base 

 of the Miocene. 



Third Phase. — Upper John Day of Diceratheres very numerous. Promery- 

 Qregon. Lower Harrison, Rosebud, and cochcerus appearing. 



Arikaree of the Great Plains of Dakota. 



Second Phase. — Middle levels (Di- Diceratheres with well-developed horns, 

 ceratherium Zone) of the John Day For- Chalicotheres in the Moropus stage, 



mation of Oregon. 



First Phase. — Upper part of White Diceratheres with very rudimentary 

 River Group of South Dakota, Leptau- horns. Cynodidis and Hyracodon 



chenia and Protoceras Zones. still surviving. Leptauchenia. 



