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



Mesatirhinus* Osborn. 



Type species M. megarhinus Earle. 



Range, Bridger C and D, Washakie A and B, now extended to include 

 Uintah A. 



The generic characters may be modified as follows: Skull length 354- 

 485 mm., frontals relatively narrow, rounded and not overhang- 

 ing the orbits, sagittal crest progressively reduced. 



M. superior, sp. nov. 



Type specimen, No. 12 188, Field Museum. (PL VI.) 



Type locality: Upper Metarhinus Sandstones,' White River divide. 



Specific characters: Skull 485x255 mm., molar series 182 mm., 

 nasals free to a point over last premolar, infra-orbital' process 

 present, arches slender anteriorly, nasals infolded at margins, 

 sagittal area expanded, canines small, P 2 and P 3 oblique to axis 

 of series. Molars relatively small, strong hypocone on M 3 , 

 posterior nares opening opposite the anterior margin of last 

 molar. 



This genus, reported for the first time from the Uintah formations, 

 is apparently indigenous to the Bridger and Washakie basins. It is 

 represented in the Field Museum collections by a single specimen — an 

 incomplete skull collected by Mr. J. B. Abbott from the top of the 

 Metarhinus Sandstones near Gilsonite Vein No. 2. The right arch is 

 wanting, together with the basi-occipital and condyles. The dentition 

 is anatomically complete excepting the incisors. 



The skull presents striking similarities with the earlier representa- 

 tives of Dolichorhinus. From the dorsal view, the nasals, facial and 

 supra-cranial regions appear very similar, though the cranial region 

 does not have the pronounced downward curve characteristic of Doli- 

 chorhinus. In the palatal view more marked differences are noticeable. 

 The premolars are more primitive, the molars smaller, and the posterior 

 narial opening is unmodified. In these characteristics the specimen 

 in hand resembles D. heterodon"\ from Upper Uintah B more closely. 

 However, it differs from that species in having a strong hypocone on the 

 last molar, and in the whole facial profile. In our present knowledge 

 of these many closely related forms, this species may be regarded as 

 the largest and most highly specialized representative of Mesatirhinus: 

 Further measurements of the type specimen are given on page 39. 



* Bull. Amer. Mus., Vol. XXIV, p. 608, 1908. 



t Douglass, Annals Carnegie Museum, Vol. VI, p. 810, 1909. 



