40 Field Museum of Natural History — Geology, Vol. IV. 



GENERIC RELATIONSHIPS 



Under the subfamily Dolichorhince are included four genera which 

 have in common the characteristics before designated. They are in 

 general low ground or river forms although certain species suggest active 

 habits. In most instances they bear evidence of incipient horn develop- 

 ment but it is doubtful if any of these forms became true horn-bearing 

 titanotheres. So far as is now known they die out with the close of the 

 Washakie and the beginning of the Upper Uintah. However, there 

 remains much to be learned of the titanotheres in their Upper Eocene 

 development. 



The most primitive genus included in this group is Mesatirhinus. 

 This is evident from the molar and premolar pattern, the oblique 

 position of the premolars in the series and the structure of the nasal 

 and frontal region. The latter characters show affinities with Palce- 

 osyops to which group the type species was first referred. Mesatirhinus 

 may therefore be regarded as standing in an ancestral relation to the 

 other members of this subfamily, though there is little evidence that 

 it actually gave rise to any of them. The relationships of the Bridger 

 and Washakie members of the genus do not come within the province 

 of this paper. 



Metarhinus, as has already been observed, may be regarded as in- 

 digenous to the Uintah Basin. Its type specimen is the earliest mammal 

 from this formation whose relationships have been determined. It 

 attains a wide dispersal and becomes the most characteristic fossil in 

 the lower subdivision of the Uintah Series. This genus in its most 

 primitive Uintah representative is not far removed from the type and 

 most primitive species of Mesatirhinus. As near as may be determined 

 from the present limited knowledge of the lowermost Uintah fauna, the 

 two type species are nearly contemporary. Mesatirhinus appears to be 

 indigenous to the Bridger and Washakie basins, Metarhinus to the 

 Uintah. Each, in its later development, is represented in the other's 

 native formation by an immigrant species. 



While the types of these two genera are similar in size and nearly 

 contemporary, Metarhinus is in many ways more highly specialized. 

 The dentition is more advanced in the general trend of titanothere 

 specialization, the nasal and facial region is likewise more specialized. 

 On the contrary it retains in the high sagittal crest an equally primitive 

 character which the most advanced species have not quite eliminated. 



Dolichorhinus appears suddenly in the Lower Uintah Series as a 

 highly differentiated form. It is earliest reported* from the Washakie 



* Earle, Osborn Op. cit. 



