NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 9 



There would seem to be little doubt that some differences observed 

 between the Bison basin faunas may be attributed to time. That there 

 is a time difference seems evident from the stratigraphic relations, and 

 the localities in the foregoing tabulation are arranged from left to 

 right in ascending order as far as I can determine, except that the 

 relative position of the west end with respect to the ledge locality is 

 entirely uncertain. The more marked differences between the various 

 localities, such as in the genera represented, would, between what ap- 

 pear to be closely related faunas, have less significance as a time fac- 

 tor than the differences observed between the most closely related 

 types. Change resulting from the evolution of certain forms, or the 

 superseding of primitive by more advanced though related types, may 

 be noted in at least three of the orders. 



Among the primates Pronothodectes is represented in both the sad- 

 dle and ledge localities, but evidently not higher, and the more primi- 

 tive Pronothodectes matthewi has been found only at the saddle level. 

 Plesiadapis is recognized at all localities, and the smaller of these, 

 P. jepseni, is best or more typically represented at the saddle and 

 ledge, whereas large P. cf. fodinatus has not been found in the saddle 

 and most of the material is from the presumably highest or Titanoides 

 locality. 



Among the Carnivora, Tricentes was encountered frequently at the 

 saddle, scantly at the ledge and not higher. Particularly striking with 

 regard to the change in Carnivora is the development of Thryptaco- 

 don. The small T. belli found in the saddle is replaced at the ledge 

 by distinctly larger T. demari, almost certainly through development 

 in situ. Material of Thryptacodon, which appears structurally a 

 little different than the foregoing and resembling more closely T. aiis- 

 tralis, is found only at the west-end and Titanoides localities, although 

 associated there with scant materials that appear to indicate one or 

 both of the other species. These latter, however, may possibly repre- 

 sent variation within a population of a single species in which the 

 mean is rapidly changing. On the other hand, T. australis may have 

 appeared from elsewhere during the later time here represented. The 

 impressive display in range of size for Claenodon is seen only at the 

 saddle, and only the typically large form compared to Claenodon 

 jcrox and possibly a single specimen of Claenodon acrogenius occur 

 at higher levels. It is possible that Tricentes and the smaller forms 

 of Claenodon may have become extinct during the interval included 

 by the Bison basin faunas. 



Most of the small condylarths are found only in the saddle or ledge 

 faunas, and there appears in these no comment-worthy differences be- 



