4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



polestes, and certain of the species are also lower Eocene. Neverthe- 

 less, this fact, together with the relative abundance of condylarths and 

 Plesiadapis, and absence of the more common Eocene forms such as 

 Hyracotherium, Homogalax, Hyopsodus, Diacodexis, Pelycodus, etc., 

 is regarded as characterizing Clark Fork time. 



It is of further interest that all the genera here recognized are also 

 either Tiffanian or, as Ectocion and Anacodon, arbitrarily distin- 

 guished from Tiffanian ancestral forms. This is regarded as further 

 characteristic of Clark Fork time, i.e., the bulk of the Clark Fork 

 fauna is a survival of a certain selection of known Tiffanian lines with 

 the appearance of very few new stems from elsewhere. It would 

 appear then that recognition of Clarkforkian time as distinct from 

 Tiffanian on a generic level is, as in comparison with the Eocene, 

 somewhat negative in character, partly depending upon the absence 

 of a number of forms comparatively common in the earlier beds but 

 presumed to have become extinct. Nevertheless, the species for the 

 most part are advanced over those of Tiffanian time and we have in 

 the Almy, for example, such forms as Plesiadapis cookei, Anacodon? 

 nexus, Ectocion ralstonensis, E., cf . osbornianum, Phenacodus almien- 

 sis and P. primaevus. From the Clark Fork beds in the Big Horn 

 Basin there may be added to this list of progressive species such forms 

 as Thryptacodon antiquus, Didymictis protenus, Haplomyliis speirianns, 

 and a species of Coryphodon. 



The further evidence given the distinctiveness of Qark Fork time 

 by the first known appearance of tillodonts and palaeanodonts calls 

 attention to the comparatively few new lines that appear, evidently 

 introduced from some other area, in contrast with the strikingly large 

 and important part of the Eocene fauna that appeared at the end of 

 Clark Fork time. This emphasizes the appropriateness and, undoubt- 

 edly, formed part of the reasoning followed in regarding Clarkforkian 

 as upper Paleocene rather than Eocene, thereby permitting factors of 

 a regional or perhaps greater importance that must have affected the 

 faunal distribution to be correlated with an important time boundary. 



SYSTEMATIC DESCRIPTION OF THE MAMMALIAN REMAINS 



PRIMATES 

 PLESIADAPIDAE 



PLESIADAPIS RUBEYI Gazin, 1942 



Plate I, figure 10 



No additional material of this species has been found since descrip- 

 tion of the original Geological Survey collection. Included is the type 



