NO. 6 PALEOCENE FAUNAS OF BISON BASIN — GAZIN I7 



Discussion. — The closeness of the resemblance between D. con- 

 cordiarcensis and D. pearcci convinces me that whatever disposition 

 is made of one, so far as the genus is concerned, the other must like- 

 wise be assigned. These two are characterized by comparatively high 

 trigonids and abbreviated molar talonids, associated with a progres- 

 sive P4, with a likewise abbreviated though basined talonid. The com- 

 bination of characters seems most closely approximated in the San 

 Jose and Knight material of Diacodon alticuspis. In Palaeictops P4 

 is comparable in progressiveness, but the talonid in this tooth and in 

 the lower molars is broad and long, comprising a greater proportion 

 of the tooth crown. The molar trigonids, though also high in certain 

 species, are relatively shorter anteroposteriorly. Prodiacodon is like- 

 wise characterized by comparatively large talonids. A small cuspule 

 about halfway down the posterior slope of P2 and P3 in Prodiacodon 

 puercensis and much better developed in species of Palaeictops is 

 not seen on the P2 of D. concordiarcensis or the P3 of D. pearcei. On 

 the other hand, an anterior cuspule, well developed and high on these 

 teeth in the latter two species, is absent or weak and low in Prodiaco- 

 don and Palaeictops. 



Among the other early leptictids, Leptacodon has a less progressive 

 P4 than Diacodon, with a strong but low paraconid and in particular 

 a comparatively weak and more posterior metaconid. Moreover, the 

 molar trigonids appear lower and the talonids comparatively larger 

 than in Diacodon. Myrmecoboides has a large P4 with paraconid for- 

 ward as in Diacodon and Palaeictops, though lower; however, the 

 greatly elongated talonids exhibited in the molars and P4 immediately 

 distinguish this genus. The abbreviation of the talonid on P4 and the 

 lower molars of Xenacodon is suggestive of Diacodon, but P4 would 

 appear to be distinctly less progressive in that the paraconid is small 

 and the talonid not basined. 



MEASUREMENTS IN MILLIMETERS OF LOWER TEETH IN TYPE SPECIMEN 



OF Diacodon pearcei, u.s.n.m. no. 20970 



Pa, anteroposterior diameter : greatest transverse diameter 1.8:1.0 



P*, anteroposterior diameter : transverse diameter of trigonid 2.4:1.4 



Ml, anteroposterior diameter : transverse diameter of trigonid 2.2:1.6 



PANTOLESTIDAE 



BIS0NALVEUS,4 new genus 



Type. — Bisonalveiis broumi, new species. 



Generic characters. — Resembling Aphronorus, but P4 much smaller 



* Bison + alvcus, basin — for Bison basin. 



