June, 1912. New Titanotheres — Riggs. 41 



formation of a somewhat later stage and as an even more highly spe- 

 cialized form. It is thus a successor to Mesatirhinus in its geological 

 range (Fig. 1), though D. longiceps occurs abundantly in the Uintah 

 formation at a level 400 feet below the horizon from which the occur- 

 rence of Mesatirhinus superior is recorded. Members of these two 

 species, though similar in size, have less in common than has the latter 

 with D. heterodon of a later stage. These similarities are sufficient to 

 suggest that Dolichorhinus probably sprang from some earlier and less 

 specialized form of Mesatirhinus. The differentiations of the former 

 may well have arisen with its adaptation to aquatic habits. 



Rhadinorhinus apparently represents a side branch from the early 

 Metarhinus stem. The facial and cranial regions and the zygomata 

 are similar. The nasals indicate an early specialization in another 

 direction. The dentition is more highly specialized than that of Doli- 

 chorhinus and in some respects resembles it. The posterior nares open 

 a little farther forward than those of Metarhinus. The mere trace of 

 an infra-orbital process also removes it somewhat from the long-nosed, 

 low-ground titanotheres. 



The broad-headed genus Sthenodectes appears in Horizon B as a well 

 differentiated form. Its highly developed dental structure, short 

 cranium and nasals offer only distant relationships with the more 

 common Lower Uintah titanotheres. It would fall into another sub- 

 family which should include the short-nosed, heavy-toothed forms which 

 were evidently ancestors of the true horned titanotheres. 



r 



ACKNOWLEDGMENT 



In the preparation of this publication Professor Osborn has accorded 

 the writer the freest access not only to the type specimens, but also 

 to the entire series of titanothere collections brought together at the 

 American Museum in the preparation of his forthcoming monograph 

 on the Titanotherida?. Dr. W. K. Gregory of the same Museum has 

 given most valuable assistance in making comparisons and in furnishing 

 unpublished data from these collections in order to facilitate identifica- 

 tions. Mr. O. A. Peterson of the Carnegie Museum has also furnished 

 unpublished data from his field observations in regard to faunal horizons 

 and other aids in establishing stratigraphic subdivisions. For these 

 courtesies and aid the writer wishes to acknowledge his obligations. 



