June, 1912. New Titanotheres — Riggs. 29 



Metarhinus earlei* Osborn. 



Type specimen, No. 13 166 Amer. Mus. 



Type locality: Washakie B and Upper Metarhinus Beds (Horizon 



A). 

 A splendid skull (No. 12 187, Field Museum, PI. VIII), two mandibles 

 (Nos. 1 21 78 and 12 189) and a distorted skull (No. 12 169), all from the 

 uppermost Metarhinus Sandstones, are referred to this species. From 

 these referred specimens, as well as from the type, the following 

 characters are derived: 



Skull, short and broad in frontal region, length 388-405 mm., breadth 

 245-255 mm. Molar series broad and low-crowned, no hypocone 

 on M 3 ; linea aspera uniting by regular curves above posterior 

 margins of zygomata to form a short, thickened sagittal crest; 

 canines slender, diastema short, P 2 oblique, P 3 and P 4 sub- 

 rectangular in outline, molars broad and low-crowned, no hypo- 

 cone on M 3 , mandible with ramus slightly curved. 



Metarhinus represents a rapidly expanding and comparatively short- 

 lived group. Its known vertical range in the Uintah is less than one 

 thousand feet. As before indicated, the genus early divides into two 

 lines which run parallel through the greater part of Horizon A and 

 disappear at the top of the heavy river sandstones. In general these 

 two phyla may be characterized as the broad-headed and the narrower- 

 headed forms. 



There is a considerable increase in size between the earlier and later 

 known forms indicated by the comparative length of skulls (352-406 

 mm) . The most evident specialization undergone during the develop- 

 ment of this genus is the reduction of the sagittal crest, the expansion 

 of the nasals distally, the strengthening of the squamosal element of 

 the arch, the appearance of incipient horns at the fronto-nasal suture, 

 the increase in size of the molar teeth and the relative reduction and 

 modification of the premolars. The incisors are so seldom preserved 

 that few deductions can be drawn from them. The canines, strong in 

 the generic type, continue so in the narrower-headed form, M . riparius. 

 In the broader-headed species, M . cristatus and M. earlei, the canines 

 become much reduced in size. The post-canine diastema is quite short 

 in all species. P 1 is a simple cone implanted by two roots; P 2 retains 

 its primitive oblique position in this series. P 3 and P 4 have become sub- 

 quadrate in outline and approach, in the broad-headed forms, the 

 transverse position in the series. The hypocone on M 2 which is prom- 

 inent in M. fluviatilis persists in the later representatives of M. riparius 



* Bull. Amer. Mus., Vol. XXIV, p. 610, 1908. 



