44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



attention to this possibility at the time he published Gidley's descrip- 

 tions. There is some difference in the stratigraphic levels attributed 

 to the two, but not as much difference as between either of them and 

 Gidleyina superior. Nevertheless, to judge by the variation in molar 

 structure noted for both Gidleyina and Ectocion, G. superior may be 

 no more than a variant of G. montanensis. The possibility also re- 

 mains that G. wyomingensis is likewise not distinct, but possible 

 synonomy here awaits demonstration that the lower premolars ex- 

 hibited in the type of G. silberlingi are atypical. 



A form which Simpson (1935c) described as Phenacodus gidleyi in 

 the Tiffany fauna has teeth only a trifle larger than in G. wyomingen- 

 sis, but the trigonid portions of the lower teeth in the type of P. gidleyi 

 represent a little greater proportion of the tooth length than in 

 G. wyomingensis. I am unable to determine whether P. gidleyi repre- 

 sents Phenacodus or Gidleyina. 



MEASUREMENTS IN MILLIMETERS OF TEETH IN SPECIMENS OF 



Gidleyina wyomingensis 



U.S.N.M. 

 No. 2079s 



Length of upper molar series, M^-M' 20.1 



M\ anteroposterior diameter : greatest transverse diameter 7.4: 9.6* 



M*, anteroposterior diameter : greatest transverse diameter 7.2:10.4 



M', anteroposterior diameter : greatest transverse diameter 5.7: 8.4 



U.S.N.M. 



No. 20790 U.S.N.M. 



(type) No. 20793 



Pj, anteroposterior diameter : greatest transverse 



diameter 5-8 : 3.5 



P4, anteroposterior diameter : greatest transverse 



diameter 6.8 : 4.7 



Ml, anteroposterior diameter : greatest transverse 



diameter 6.5 : 5.2 6.6 : 5.2 



Mj, anteroposterior diameter : greatest transverse 



diameter 6.8 : 5.6 



Ma, transverse diameter of trigonid 4.8 



" Approximate. 



PHENACODUS? BISONENSIS,'" new species 

 Plate 10, figures 1-3 



Type. — Right maxilla with P*-M^ U.S.N.M. No. 20564, and prob- 

 ably a left maxilla with M^ and M^ believed to be from the same 

 individual. 



Horizon and locality. — Bison basin Tiffanian, vicinity of saddle lo- 

 cality at south rim of Bison basin, sec. 28, T. 27 N., R. 95 W., Fre- 

 mont County, Wyo. 



1^ Named for the Bison basin. 



