June, 1912. New Titanotheres — Riggs. 27 



Metarhinus Osborn 



The genus Metarhinus is based upon a series of specimens from the 

 Uintah Metarhinus Beds. It has for its type a skull (Am. Mus. No. 

 1500) from the lowermost horizon* of these beds. Two other species, 

 M. earlei and M. diploconus have been referred to this genus. f The 

 former species from the Upper Washakie Beds is recognized from the 

 Upper Metarhinus Beds of the Uintah. The differences between the 

 type of M. diploconus and other species more directly in line with the 

 generic type appear to warrant its removal from this genus. § 



The Field Museum collection includes a more complete series of 

 specimens referable to Metarhinus than has hitherto been known. 

 Among these are seven adult skulls, and one of a young individual, 

 together with two associated and various isolated mandibles. These 

 specimens come from higher levels and represent more advanced stages 

 of development than the type species. 



From a comparative study of all the materials referred to Metarhinus 

 the genus may be characterized as follows: 



Type species, M. fluviatilis.% 



Range: Uintah Metarhinus Beds (Horizon A) and Washakie B. 

 Generic characters: Small titanotheres (skull length 355-415 

 mm.) having orbital region unusually prominent, nasals expanded 

 distally and free to a point above the anterior margin of the orbit, 

 incipient horn-cores at fronto-nasal suture, posterior nares open- 

 ing opposite second molar, sagittal crest more or less reduced, 

 molars with low crowns, hypocone on M 3 present or reduced. 

 Mandible with narrow, straight ramus, first lower premolar 

 single-rooted or absent. 



The genus includes two phyla, which are apparently distinct but so 

 far as is now known merit no more than specific rank. The first phylum 

 includes M. fluviatilis from the Lower, and M. riparius from the Upper 

 Metarhinus Beds. These are clearly indigenous types known from the 

 Uintah only. In their dental characters they are the more primitive. 

 The canine is large, the premolars more diagonal and the hypocone on 

 M 3 present, often prominent. The molars in adults are usually deeply 

 worn. The second phylum includes forms relatively short-headed, 

 broad in the frontal region, with more rectangular premolars, broad 



* Fide Peterson. 



t Bull. Amer. Mus., Vol. XXIV, p. 610. 



§ Vide infra. 



t Bull. Amer. Mus., Vol. XXIV, p. 609, 1908. 





