NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 2/ 



P4 in U.S.N.M. No. 20983 measures 6.1 mm. long by 3.7 mm. wide. 

 Ml in this specimen is 7.3 by 5.4 mm. 



An isolated upper molar, probably M^ (U.S.N.M. No. 21003), 

 would also appear to represent a species of Chriacus about the size of 

 C. pclvidens. The outer styles of this tooth are not noticeably de- 

 veloped, but lingually the cingulum carries a prominent hypocone and 

 a likewise prominent though less developed protostyle at the antero- 

 lingual margin of the tooth. This tooth measures 6.3 mm. long by 

 8.8. mm. wide transversely. 



CHRIACUS, sp. 

 Plate 5, figure 3 



A single upper molar from the saddle locality, U.S.N.M. No. 21019, 

 presumably M-, is much smaller, approximately 25 percent less in 

 general proportions than the M- discussed in the foregoing section. 

 In size it would appear to be more nearly comparable to Chriacus 

 truncatus, approximately that of Thryptacodon belli. The rectangu- 

 lar appearance of this tooth and the prominence of the anterointernal 

 cusp or protostyle would seem to remove it from consideration as a 

 form of Thryptacodon. The tooth measures 5.2 mm, long by 6.3 mm. 

 wide transversely. 



THRYPTACODON, cf. AUSTRALIS Simpson, 1935 

 Plate 6, figure 5 



A fragmentary left mandibular ramus, including P4, Mi, and M3, 

 in the collection obtained by the University of Wyoming (No. 1076) 

 from the Titanoides locality, in details of the teeth closely approxi- 

 mates that of the species of Thryptacodon named by Simpson ( 1935c) 

 from the Tiffany beds of Colorado. The teeth are a trifle larger than 

 in the type as may be seen from the dimension Simpson has given, but 

 the rudimentary condition of the mctaconid on P4 and the prominently 

 isolated hypoconulid on M3 suggest possibly a closer relationship to 

 Thryptacodon australis than to T. demari described as new in the 

 following section. 



The trigonid of M3 in U. Wyo. No. 1076 seems rather broadly 

 basined and the paraconid distinctly weak. Moreover, the hypo- 

 conulid, in addition to being prominent, is rather distinctly set off 

 from the entoconid, and there is a low crest between the entoconid and 

 hypoconid. The hypoconulid portion of M3 is believed to be highly 

 variable in Thryptacodon, but T. australis material observed shows 

 this cusp rather better defined than in much of the Wasatchian 



