10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



tween the two faunas ; however, a single specimen of Haplaletcs en- 

 countered at the Titanoidcs locality is of a surprisingly larger form 

 than the Ilaplaletes represented at the saddle. On the other hand, a 

 single specimen from the saddle referred tentatively to Litolestcs 

 lacunatus is scarcely different than the typical material from the Tita- 

 noides locality. Among the larger condylarths the form described as 

 Gidleyina wyomingensis from the ledge and west-end localities may 

 be a little more progressive than indicated by material referred to it 

 from the saddle. Abundant Phcnacodus? bisonensis is almost re- 

 stricted to the saddle level although two specimens came from the 

 ledge. This seems replaced by a considerably larger, at the same time 

 much more rare, species from the ledge and higher. The span of time 

 represented by the Bison basin faunas may have witnessed the extinc- 

 tion of such forms as Promioclacnus (which includes much that had 

 been grouped before under Ellipsodon), Protoselene, and Litomylus. 

 It is not known to what extent the times of Caenolambda and Tita- 

 noides may have overlapped. Typical materials of each were found 

 as single specimens at the saddle and Titanoides localities respec- 

 tively. Undetermined pantodont tooth fragments were collected at all 

 localities. 



AGE AND CORRELATION OF THE FAUNAS 



In consideration of the age or ages represented by the Bison basin 

 faunas we may deal first with that represented at the saddle locality, 

 rather clearly the oldest of the four levels. In regarding this as Tif- 

 fanian somewhat greater emphasis is given to the appearance of forms 

 known to characterize later horizons than to the presence or survival 

 of older genera. For example, Plesiadapis, Thryptacodon, Litolcstes, 

 Gidleyina, and Phcnacodus may be regarded as Tiffanian in first ap- 

 pearance, whereas Pronothodcctes, Tricentes, Clacnodon, Promioclac- 

 nus, Litomylus, Haplaletcs, and Protoselene have been rather generally 

 thought to be Torrejonian. Species represented of certain of the lat- 

 ter genera are not clearly separable from those of the Torrejonian 

 levels in the Nacimiento and Fort Union and might be regarded as 

 long lived, but others in this group such as Pronothodcctes simpsoni, 

 Clacnodon acrogcnius, Litomylus scaphicus, and Haplaletcs pelicatus 

 are distinctly advanced. 



The interpretation that there is an admixture of materials of rather 

 different horizons at the saddle locality, though not impossible, may 

 be discarded as a serious possibility inasmuch as nearly all the genera 

 of both aspects are found together at the ledge locality where collect- 

 ing was limited to a narrow zone above a ledge and very near the top 

 of the escarpment. Moreover, among the Tiffanian genera repre- 



