Peterson: The Osteology of Promerycochoerus. 217 



equally justifiable to say that the Lower Oligocene has a still charac- 

 teristically Upper Eocene fauna, as to say that the Lower Harrison has 

 a "characteristically Oligocene" fauna? Have we not in the Lower 

 Oligocene the Hyracodonts, the Titanotheres, the Metaniynodonts, the 

 Hya^nodonts, and the Anthracotheres, which do not occur in the 

 Lower Miocene (Monroe Creek and especially the Lower Harrison 

 beds)? \\'ould not the absence in the Lower Miocene of these forms 

 of the Lower Oligocene correspond to some extent to the characteristic 

 absence in the Lower Oligocene of such forms as Eobasileus, Achano- 

 don, Mesonyx, and other archaic forms found in the Upper Eocene 

 (Uinta) and entirely absent in the Oligocene? 



It is after all only in a rather vague and superficial manner that we 

 can correlate the formations of widely separated localities. We have, 

 indeed, as Osborn has clearly pointed out, the Lower Miocene clearly 

 indicated in Europe by the sudden appearance of a number of 

 strange forms, such as the proboscidians, dinotheres, short-limbed 

 teleoceran forms of the Rhinocerotidae, the antelopes (Protragoceras) 

 and other forms; while in North America on the other hand the transi- 

 tion from the Oligocene to the Miocene, so far as evidence is afforded 

 by their faunae, appears to have been gradual and not marked by the 

 sudden introduction of many new forms. 



It is very well agreed among geologists as well as paleontologists 

 that there are well-marked lithologic differences between the upper- 

 most Oliogocene (Leptauchenia beds) and the succeeding formations 

 (Gering and Monroe Creek beds) in western Nebraska and Wyoming; 

 differences so great as to justify the belief in an important geologic 

 interval. This view is not of recent origin. Although the remains 

 supposed to have been found in Dr. Hayden's "Horizon E" of this 

 general section do not seem to agree with material subsequently found 

 in the Monroe Creek, Rosebud, and Lower Harrison beds, it is quite 

 evident from his description that these deposits, especially along the 

 upper waters of the Niobrara River, answer well to his statements. 

 After all in many instances we are only dealing with convenient imag- 

 inary lines in separating geologic formations as well as phyletic series 

 in paleontology. Instead of establishing the point of division between 

 the Oligocene and the Miocene by running a line through a sedimentary 

 mass of quite uniform character, as in the Upper and Lower Harri- 

 son, I personally prefer, being guided by lithological as well as faunal 

 features, to regard the beds (Monroe Creek and Lower Harrison) over- 

 lying the Leptauchenia beds in Nebraska as belonging to the Lower 



