NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN 29 



tion of the type, the talonid basin of M3 is confined posteriorly by a 

 low crest between the hypoconid and entoconid and the hypoconulid 

 is sharply separated from the entoconid. It should be noted, however, 

 that an approach to this condition is made in certain specimens of T. 

 detnari, hence possibly not of diagnostic significance. 



Differences from T. antiquus, in addition to the development of the 

 metaconid on P4, include a little less difference in width between M2 

 and the trigonid of M3, with M3 relatively much shorter. Moreover, 

 teeth of Thryptacodon demari show a cingulum, usually discontinuous, 

 external to the hypoconid, but it appears not to be developed external 

 to the protocone to the extent seen in T. antiquus. Also, it is not nearly 

 so expanded posterior to the hypoconid on M3. 



A fragmentary maxilla, U.S.N.M. No. 20984, referred to this 

 species, has M^ and M- preserved, and a second maxilla, U.S.N.M. 

 No. 20992, has only M^, Also, there is an isolated M^ in the Uni- 

 versity of Wyoming collection which may represent this species. 

 M^ exhibits an anteriorly projecting and weakly cusped parastyle. 

 The cingulum is evenly continuous around the anterior and lingual 

 portions of the tooth and the hypocone is a simple conical cusp rising 

 from the cingulum posterointernal to the the protocone. There is no 

 protostyle and there are no particularly distinct accessory cuspules 

 adjacent to the hypocone as observed in the Eocene materials. M^ 

 lacks the distinctive parastyle, and in No. 20984 (but not in No. 

 20992) there is a very rudimentary protostyle anterolingual to the 

 protocone where the cingulum is somewhat more sharply deflected 

 around the margin of the tooth than in M^ As in M\ however, there 

 are no clearly distinguishable accessory cuspules adjacent to the 

 hypocone. 



The right M^ described by Simpson (1928) as Thryptacodon 

 pseudarctos in the Bear Creek Paleocene fauna of southern Montana 

 is larger and apparently has a more robust protocone than in No. 20984 

 considered to be Thryptacodon demari. The type, T. pseudarctos, 

 measures 6.9 by 8.6 mm. Measurements of teeth in T. demari are in- 

 cluded with those of T. belli. 



THRYPTACODON BELLI,^o new species 

 Plate 6, figures i, 4 

 Type. — Right ramus of mandible including M1-M3, U. of Wyo. 

 No. 1045. 



10 Named for Wallace G. Bell, engaged in the geologic mapping of the Bison 

 basin area. 



