NO. 6 PALEOCENE FAUNAS OF BISON BASIN GAZIN II 



sented at the saddle an early stage of development seems clearly indi- 

 cated in Plesiadapis jepseni, Thryptacodon belli, and the material re- 

 ferred to Gidlcyina ivyomingensis, as well as in the close approach 

 that Phenacodus? bisonensis makes to Tctraclaenodon. 



With due regard to the presence of forms of older aspect in the 

 fauna an age assignment of early or lower Tiffanian rather than later 

 Torrejonian seems indicated. This is further supported by the re- 

 semblance or similarity that persists between the Bison basin faunas, 

 although above the saddle and particularly above the ledge locality 

 the Torrejonian aspects appear to be lost. Direct comparison of the 

 saddle fauna with that of the Melville is difficult because of the spar- 

 sity of Carnivora and condylarths so well represented in the Bison 

 basin ; nevertheless the saddle level may not be much older than the 

 Melville, generally regarded as lower Tiffanian. 



The fauna from the Titanoides locality is rather limited, but evi- 

 dence is seen for a somewhat more typical Tiffanian stage. The pres- 

 ence of Titanoides primaevus suggests a definite relationship only with 

 a horizon in the type Fort Union in western North Dakota, although 

 somewhat smaller forms from the Melville and Silver Coulee have 

 been referred to this genus, as well as "Sparactolambda" looki from 

 the DeBeque beds. A large species of Plesiadapis regarded as closest 

 to P. fodinatiis from the Silver Coulee level of the Polecat Bench se- 

 ries suggests an equivalent horizon in Tiffanian time, but the Bison 

 basin form in at least one individual retains a second lower premolar 

 not seen in any of the typical P. fodinatus material. Material close to 

 Plesiadapis jepseni also occurs at the Titanoides locality, suggesting 

 a closer tie with the older levels in the Bison basin. The Thryptacodon 

 here is evidently to be compared closely with that from the Tiffany 

 beds in Colorado. The presence of Claenodon cf. ferox is not signi- 

 ficant in view of the still later occurrence of this genus in beds of 

 Clarkforkian age elsewhere. Haplaletes serior would possibly suggest 

 a comparatively early horizon, but this species is so much larger than 

 that from the Lebo, or even than the form described from the saddle 

 locality in the Bison basin, that its significance seems lost. Litolestes 

 is a Tiffanian genus here represented by a species which appears to 

 differ only in being of larger size than that from the Melville and 

 much larger than the Silver Coulee genotype. Gidleyina and the mate- 

 rial of the comparatively large form of Phenacodus'^ at the Titanoides 

 locality suggest little other tlian Tiffanian. 



From the foregoing it would seem that the Titanoides locality fauna, 

 in the absence of forms of Torrejonian aspect, might be regarded as 

 somewhat higher in the Tiffanian than Melville but, from considera- 

 tion of the primates only, possibly not so late as Silver Coulee. 



