242 



BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



Horizon and locality. — -Gidley Quarry, Fort Union, Middle Paleocene 

 horizon. Crazy Mountain Field, Mont. 



Diagnosis. — Sole known species of genus. Dimensions given ir 

 table 54. 



Discussion. — This is the rarest of the four species of hyopsodontids 

 in the quarry, but it is distinctive and fairly well known. 



Figure 66.—Litom.ylus dissentaneus Simpson, U.S.N.M. no. 9425, left lower jaw: a, Crown view; 6, internal 



view. Four times natural size. 



Figure 67.- 



-Litomylus dissentaneus Simpson, U.S.N.M. no. 9557, with tooth in outline from U.S.N.M. 

 no. 9580, right M'-^: a, E.xternal view; 6, crown view. Four times natural size. 



No upper teeth anterior to P* are known, and P* is represented only 

 by an uncharacteristic fragment. M^~^ have sharp, subequal, nearly 

 conical paracone and metacone. The protocone is likewise sharp and 

 smaller than in the other species of this group. The conules are large 

 and equal. The external cingulum is sharp and even forming equal 

 angulations, rather than distinct cuspules, at the parastylar and meta- 

 stylar corners. There is no mesostyle. The hypocone is larger than 

 in any known contemporaneous species and is quite distinct from the 

 protocone and equally internal, but smaller. M^ is markedly trans- 

 verse and is triangular, not rounded or oval, without a hypocone but 

 with a sharp, distinct metacone. 



P 3_4 are long, low, narrow, trenchant teeth, unlike any others known 

 in this family. Each has a rudimentary, median, basal paraconid. 

 The talonids are poorly developed in each case and have only a single 

 posteromedian cuspule and a very rudimentary posterointernal basin. 



