NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 45 



Specific characters. — Size very close to that of Phenacodus ahnien- 

 sis, about intermediate between that of PJienacodus? grangeri and 

 Phenacodus? rnattheivi of the Tiffany beds. P^ and P* with tritocone 

 distinct, but much less progressive than in P. almiensis. Mesostyle 

 prominent on M^ but variable on M^. Lower premolars comparatively 

 simple and unprogressive. P'* trigonid with paraconid low and for- 

 ward, and talonid weakly basined, with entoconid generally distinct 

 though small. 



Discussion. — Approximately 30 specimens of this form are at 

 hand and nearly all are from the vicinity of the saddle locality. Two 

 specimens, however, were secured from the ledge locality statigraphi- 

 cally a little higher. 



Uncertainty exists as to whether the species represented by this 

 material should be referred to Phenacodus or to Tetraclaenodon. Its 

 allocation to Phenacodus is entirely arbitrary and scarcely more than 

 an impression. As noted by Granger (1915), there are actually no 

 clear-cut characters by which the genera may be separated. Although 

 there are differences between them in degree of development for a 

 number of characters, they are in the nature of average differences, 

 lacking in the consistency generally expected at the generic level. 

 Granger attempted a definition based on the development of the 

 mesostyle, but certain upper molars of Tetraclaenodon puercensis 

 show a rather surprising prominence in this style. The shift of the 

 metaconule posteriorward would seem evident for Phenacodus pri- 

 maevus but not diagnostic for such Paleocene forms as Phenacodus 

 almiensis or Phenacodus"? grangeri. I note a decreasing prominence 

 of the protoconule and metaconule with respect to the primary cusps 

 in rising above the Torrejon level, to the extent that in some 

 Wasatchian material of Phenacodus the metaconule is entirely missing 

 on M- and M^. Nevertheless, this is variable in populations of the 

 better known species of Phenacodus as well as in Tetraclaenodon 

 puercensis, and, like the increasing significance of the tritocone of the 

 upper premolars, is a difference in degree not readily defined. 



The lower teeth do not appear to present characters of significance 

 on a generic level. Certainly the development or reduction of the 

 paraconid is too highly variable. The talonid of P4 would seem to 

 become more molariform in time and the entoconid better developed 

 but this cusp is occasionally prominent in material of Tetraclaenodon 

 puercensis, and a decidedly primitive appearing P4 structure has been 

 observed in material representing certain of the smaller species of 

 Phenacodus in the Eocene. 



Phenacodus? bisonensis would appear to be Tetraclaenodon in the 

 subordinate appearance of the tritocone on P^ and P* and its proximity 



