FORT UNION OF CRAZY MOUNTAIN FIELD, MONT. 119 



dence suggests an undifferentiated group from which these late insec- 

 tivores arose, but annectant types are lacking and knowledge is too 

 incomplete to test this hypothesis adequately. Matthew (1918) has 

 also pointed out that Nyctitherium itself may be a chiropteran, and 

 this may be true of the whole group, or it may be a primitive complex 

 allied or ancestral to all three groups, Talpoidea, Soricoidea, and Cliir- 

 optera, although there is reason to suppose that these three groups 

 were already distinct before the end of the Paleocene. It is likewise 

 possible that two or all three of these groups had acquired their skel- 

 etal specializations but not their dental characters in the Paleocene 

 and that the nyctitheriids are an artificial assemblage based on dental 

 resemblance only. Despite this possibility, it seems most practical to 

 continue to associate them in this family until some broader basis for 

 classification becomes available. 



Figure 10.— Stilpnodon sjmplkidens Simpson, U.S.N.M. no 9629, left lower jaw: a, Crown view; 6, internal 



view. Five times natural size. 



In the National Museum collection only one nyctitheriid specimen 

 is available, but it represents a distinctive genus, Stilpnodon. It 

 is not very close to any other known genus, and its reference to the 

 family is not certainly established, but it is most conveniently placed 

 here. It shares with Leptacodon munusculum the distinction of being 

 the smallest known mammal of this fauna. 



Genus STILPNODON Simpson 



Stilpnodon Simpson, 1935d, p. 229. 



Type. — Stilpnodon simplicidens Simpson. 



Distribution. — Middle Paleocene, Fort Union, Mont. 



Diagnosis. — P4 with very high, slender main cusp, minute rudimen- 

 tary anterior basal caspule, no metaconid, simple nonbasined talonid 

 with one cuspule. M3 reduced, with distinct, low, nearly median 

 paraconid, trigonid erect and moderately elevated above talonid, 

 protoconid large, trigonid nearly as long as talonid, talonid short and 

 much narrower than trigonid. 



119212—37 9 



