FORT UNIOX OF CRAZY MOUNTAIN FIELD, MONT. 107 



phyletically distinct from any other known genus. Picrodus is really 

 of unknown affinities and is placed in the Insectivora only in default 

 of other evidence. 



Family ?DELTATHERIDIIDAE Gregory and Simpson, 1926 

 Subfamily Didelphodontinae Matthew, 1918 



Matthew placed this subfamily in the Leptictidae, pointing out 

 that the affinities of Didelphodus are uncertain, but that it "can not 

 easily be placed in any other family." Except for the general state- 

 ment that "the teeth are in many respects not unlike those of the 

 Leptictidae", he gave no definite reason for placing Didelphodus in 

 that family, where it was decidedly anomalous. In defining the 

 Deltatheridiidae, Gregory and Simpson (1926) stated that Didelphodus 

 might belong in that group, and I still later (Simpson, 1928) gave in 

 somewhat more detail the reasons for this tentative assignment. 



Gelastops of the present fauna is sufficiently close to Didelphodus 

 to warrant their tentative collocation in one subfamily but, as brought 

 out more fully in describing Gelastops, this is not certain. In any 

 case Gelastops is poorly known and adds little to evidence for the 

 affinities of this group. Its more aberrant premolar structure, still 

 m.ore striking in the probably allied Acmeodon, does to some extent 

 argue against close affinities with the much more primitive Cretaceous 

 forms but this may merely indicate an incipient line of specialization 

 within the Deltatheridiidae. The data do not warrant a more positive 

 conclusion. On present evidence it seems well to retain Matthew's 

 subfamily Didelphodontinae, for Didelphodus, Gelastops, Acmeodon, 

 and probably Phenacops. This necessitates the proposal of a new 

 subfamily Deltatheridiinae, defined, among other characters, by the 

 less progressive premolars, less separated paracone and metacone, and 

 narrower talonids. 



Genus GELASTOPS Simpson 



Gelastops Simpson, 1935d, p. 227. 

 Emperodon Simpson, 1935d, p. 229. 



Type. — Gelastops parous Simpson. 



Type o/ Emperodon. — Emperodon acmeodontoides Simpson. 



Distribution. — Middle Paleocene, Fort Union, Mont. 



Diagnosis. — Canine large and erect. P4 intermediate in structure 

 between Didelphodus and Acmeodon, with paraconid high on crown, 

 metaconid nearly as high as protoconid and partly confluent with 

 latter, a vertical crest descending posteriorly from the metaconid and 

 another from the protoconid, and a small, bicuspid, basined talonid. 

 Molars leptictid or didelphodontine, paraconids large and more inter- 

 nal than in Prodiacodon or similar leptictids, trigonids elevated, that 



