46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



to the primary cusp, and in this respect is certainly distinct on a specific 

 level from the Clarkforkian materials, such as Phenacodiis almiensis. 

 Nevertheless, these teeth show deuterocone portions more suggestive 

 of Phenacodus, and P^ has a rather conspicuously developed postero- 

 internal talon basin not observed in U, S. National Museum Torrejon 

 materials. P.? bisonensis, moreover, resembles P. almiensis in the 

 slightly more crested appearance of the upper molar cusps and in the 

 lesser significance of the conules in comparison with Tetraclacnodon 

 puerccnsis. The mesostyle of the upper molars is distinct and moder- 

 ately prominent in all referred materials ; however, in the type speci- 

 men, although prominent on M^, it is very weak on M^. 



MEASUREMENTS IN MILLIMETERS OF TEETH IN SPECIMENS OF 



Phenacodiis ? bisonensis 



U.S.N.M. 

 U.S.N.M. No. 20564 



No. 20566 (type) 



P', anteroposterior diameter : transverse diameter ... 7.8 : 6.5 



P*, anteroposterior diameter : transverse diameter ... 7.2 : 8.0 8.3 : 9.2 



M\ anteroposterior diameter : greatest transverse 



diameter 9.4 : ii.o 



M'', anteroposterior diameter : greatest transverse 



diameter 9.4:12.8 



U.S.N.M. U.S.N.M. 



No. 20567 No. 20569 



Length of lower molar series 28.7 



Pa, anteroposterior diameter : greatest transverse 



diameter 7.7 : 4.6 



P4, anteroposterior diameter : greatest transverse 



diameter 8.8 : 5.7 



Ml, anteroposterior diameter : greatest transverse 



diameter 8.9:7.6 8.2:7.3 



M2, anteroposterior diameter : greatest transverse 



diameter 9-5 " : 74 9.6 : 8.2 



Ms, anteroposterior diameter : greatest transverse 



diameter 10.5:7.2 



" Approximate. 



PHENACODUS? sp. (large) 



Plate 10, figures 4, 5 



A fragmentary right mandibular ramus with Mg, U.S.N.M. 

 No. 21025, ^^ isolated P', M', and an incomplete lower molar (the 

 latter two in the collections of the University of Wyoming) are of a 

 species much larger than Phenacodus} bisonensis. The range of size 

 represented in materials of P.? bisonensis is surprisingly limited, cer- 

 tainly in comparison with such forms as T. puerccnsis and P. primac- 

 vus, so that the teeth here indicated as of a distinct species stand out 



