PETERSON: THE AMERICAN DICERATHERES. 401 



species which have been proposed. Whether the views expressed in the work 

 upon the Diceratheres here offered shall prove to be conclusive, can only be ascer- 

 tained in the light of the future. The improbability of having been able to reach 

 an absolutely final conclusion is abundantly realized by the writer. 



Stratigraphy. 



Since the earliest descriptions of the European forms of the Diceratheres by 

 Pomel (1853), Duvernoy (1854), and the final determination of the genus by 

 Marsh (1875), many papers treating of the Rhinocerotidse have appeared in America 

 as well as in the Old World. Through the studies of Professor Osborn and Mr. 

 Hatcher, based upon some early American forms, we learn that the phylum Dicera- 

 therince had already acquired incipient nasal horns in the White River Oligocene 

 of South Dakota. It is now known that the forms, not alone of Diceratherium 

 from the succeeding John Day beds, but all other mammalian remains available 

 for comparison from the same horizon of the John Day in which Diceratherium 

 is found, represent an earlier facies than those from the Nebraska-Dakota Miocene. 



In order to give conveniently a clear view of the stratigraphic correlation 

 the diagram on page 402 is inserted. 



The Oligocene in South Dakota, as is well known, is much more extensively 

 developed than in Nebraska. It comprises, besides the three usually recognized 

 faunal zones, the Titanotherium beds (= Chadron beds), the Oreodon, and the 

 Leptauchenia beds (= Brule beds), two other easily recognized divisions, one the 

 Metamynodon beds included in the Brule beds, and the other, the Protoceras 

 sandstones, both in the Leptauchenia clays which arose from deposits made by 

 streams. The Miocene section of South Dakota falls into two (Lower and Upper 

 Rosebud beds), instead of the four divisions, recognized in Nebraska. The four 

 divisions of the Nebraska Miocene comprise the Gering, the Monroe Creek, the 

 Lower and the Upper Harrison. The latter is regarded by the writer as the base 

 of the Middle Miocene. The lower portion of the John Day beds may be regarded 

 as of transitional character and should therefore be classed as either uppermost 

 Oligocene or lowermost Miocene, the only difference being that they are not 

 separated from the INIiddle John Day beds by any apparent stratigraphic break. 

 The Mascall beds of the John Day are somewhat later than the Upper Harrison 

 beds of Nebraska, as indicated by a comparison of the faunae. 



We know the earlier progenitors of Diceratherium less clearly, though it is 

 held that Ccenopus occidentalis (Leidy) from the middle Oligocene and Ccenopus 



