NO, 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 43 



Specific characters. — Size of P4 and molars close to that in Gid- 

 leyina silherlingi. P3 much smaller and with simple posterior median 

 crest and single talonid cusp. Paraconid of lower molars variable but 

 generally low and weak, Trigonid not so prominently basined as in 

 Gidleyina superior. Upper molars with prominent styles, and crests 

 of protocone distinctive. 



Discussion. — Gidleyina zvyomingensis is better represented in the 

 more westerly and stratigraphically somewhat higher levels than at 

 the saddle locality. The type is from the locality at the west end of 

 the basin, as are about eight other specimens, although most of these 

 are isolated upper and lower teeth. The material from the ledge lo- 

 cality seems entirely similar to that from the west-end locality, and 

 among the 10 specimens from the ledge is the upper dentition 

 (U,S.N,M. No. 20795) figured in plate 9 and about three jaws with 

 two or more molars. About five specimens from the saddle locality, 

 including portions of upper and lower dentitions, might well represent 

 a slightly smaller variant, though probably not specifically distinct 

 from that represented in the material from the more westerly collec- 

 ing sites. 



Doubt may be logically entertained as to the advisability of recog- 

 nizing Gidleyina as distinct from Ecfocion. Comparison of the Tif- 

 fanian materials with the genotype of Ecfocion, E. oshornianum, from 

 the lower Eocene would seem to justify separate recognition but, as 

 may be expected, the Clarkforkian materials, particularly those from 

 the Almy, in no way simplify this arrangement. As noted by Simp- 

 son, the upper premolars in Gidleyina are less progressive and the 

 upper molars show better development of crests from the protocone 

 to the protoconule and metaconule. No upper premolars appear to be 

 included in the Bison basin collections but the molars exhibit the 

 protocone crests as mentioned above, and in comparison with G. mon- 

 tanensis have perhaps somewhat better developed external styles. 

 P4 in both the Montana and Wyoming Gidleyina material would ap- 

 pear distinctive when compared with Eocene material of Ecfocion, 

 principally in that the trigonid is elongate in comparison with the 

 talonid length, whereas in E. oshornianum this relationship is rather 

 generally reversed with the trigonid often short and broad and the 

 talonid usually, though not invariably, better developed. Moreover, 

 in some individuals of E. oshornianum the talonid of P4 looks quite 

 molariform, with a surprisingly well developed entoconid. 



With regard to the forms of Gidleyina known from the Melville unit 

 of the Montana Fort Union, I strongly suspect that Gidleyina sil- 

 herlingi is a synonym of G. montanensis. Simpson (1937b) called 



