FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 121 



partial jaw from the Aliddle Paleocene to Palaeosinopa. 1 have 

 described a genus, Bessoeceter, from the Upper Paleocene (Simpson, 

 1936b). The present fauna includes a distinctive species that seems 

 surely to be closely allied to or in the Palaeosinopa-Pantolestes line 

 and is tentatively referred to Bessoecetor, thus definitely projecting 

 the typical pantolestid phylum back into the Middle Paleocene. It 

 also includes in Ajphronorus an ally of Pentacodon, represented by 

 much better material than is that Torre j on genus and much improving 

 our knowledge of this small group. 



The probable relationship of Pentacodon and Ajphronorus, on one 

 hand, and Bessoecetor, Palaeosinopa, and Pantolestes, on the other, is 

 confirmed and strengthened by the present evidence. In both the 

 lower jaw has a large semiprocumbent canine followed by three small 

 elongate premolars of similar pattern in the two groups and then by 

 a relatively enlarged premolar, the pattern of which is, however, 

 unlike in the two lines. The molars, upper and lower, are of almost 

 exactlj^ similar pattern, with only such minute differences as are 

 encountered among veiy closel}- allied genera. The posterior mental 

 foramen is beneath Mi in both cases and the other known osteological 

 characters, although few and not veiy distinctive, are generally 

 similar. 



The only characters opposing such relationships are those of P^4, 

 which are more and differently specialized in Pentacodon and Aph- 

 ronorus than in Bessoecetor or even the much later genus Pantolestes. 

 These marked differences do not seem to exclude a family relation- 

 ship, but they do show that two divergent and sharply defined groups 

 are included, especially when the distribution of the various genera is 

 taken into account. In the present state of knowledge it seems 

 convenient to formalize this distinction by placing the known genera 

 in two subfamilies, the Pantolestinae with Bessoecetor, Pantolestes, 

 Palaeosinopa, and perhaps some less well known forms, and the 

 Pentacodontinae with Pentacodon and Aphronorus. For the present 

 these are immediately and adequately distinguishable from the 

 characters of P*4 as given in the diagnosis below. 



Pantolestinae, new subfamily 



Type. — Pantolestes Cope, 1872. 



Distribution. — Middle Paleocene to Middle Eocene, North America. 

 Lower Eocene, Europe. 



Diagnosis. — P*4 relatively little enlarged. P4 compressed, no 

 metaconid, semishearing, heel essentially unicuspid and little or not 

 basined. P^ with compressed amphicone, no metacone, low, sharp 

 styles, small protocone without cingula. 



