234 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



genus somewhat uncertain. The structure of P4, the extreme reduc- 

 tion of M3, and perhaps some less clear details are peculiar with respect 

 to E. lemuroides or E. acolytus, and the fact that the genus is known 

 principally from these atypical species may mean that its true nature, 

 as based on E. inaeguidens, is now seriously misunderstood. 



In general aspect, E. aquilonius resembles some of its associates 

 such as Litaletes disjundus more than it does Ellipsodon inaeguidens, 

 but in structural detail it seems closer to Ellipsodon acolytus and is 

 conservatively associated mth that species generically (rather than 

 definitely with the type of Ellipsodon). 



ELLIPSODON AQUILONIUS Simpson 



Figures 62, 63 

 Ellipsodon aquilonius Simpson, 1935d, p. 242. 



Type. — U.S.N.M. no. 9280, right lower jaw with P3-M3 and alveoH. 

 Collected by A. C. Silberling. 



Paratype. — U.S.N.M. no. 9567, right upper jaw with P^-M^. 

 Collected by Dr. J. W. Gidley. 



Horizon and locality. — Types from Gidley Quarry, surely referable 

 specimens from Silberling Quarry and one, more doubtful, from Loc. 

 51, Fort Union, Middle Paleocene horizon, Crazy Mountain Field, 

 Mont. 



Diagnosis. — Close to E. acolytus in size and structure, but teeth 

 generally slightly slenderer, P4 relatively shorter and with metaconid 

 more distinct. M^a only moderately reduced. P^ without protocone. 

 M^~^ with rudimentary hypocone not connected to protocone apex. 

 Measurements given below. 



Discussion. — This is one of the commonest species in the quarries 

 and is represented by a fine series of specimens revealing its dental 

 morphology and variation in detail, although in no case is the anterior 

 dentition preserved. 



The number of incisors is unknown. The post-incisive dentition 

 was complete numerically. 



The upper canine and P^ are unlaiown. P^ is a small simple tooth 

 with a small anterior and a larger posterior root. The crown is com- 

 pressed, trenchant, with a median cusp, minute posterior cuspule, and 

 posterointernal cingulum. P^ has three roots but is longer than wide 

 and has only one distinct cusp, which is central and is triangular in 

 section, with minute anterior and posterior basal cuspules and a 

 sharp, continuous, but not cuspidate internal cingulum, stronger on 

 the posterointernal than on the anterointernal face. P* is wider 

 than long and has a strong protocone, which is, however, lower than 

 the amphicone. There is no separate metacone. The amphicone is 

 triangular and has a sharp posterior and a weak anterior crest from 



