120 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



STILPNODON SIMPLICIDENS Simpson 



Figure 19 

 Slilpnodon simplicidens Simpson, 1935d, p. 229. 



Type.— U.S. ISiM. no. 9629, left lower jaw with P3_4, M3, and 

 alveoli. 



Horizon and locality. — Gidley Quarry, Fort Union, Middle Paleo- 

 cene horizon. Crazy Mountain Field, Mont. 



Diagnosis. — Sole known species of genus. P4 length, 1.0 mm. M3 

 length, 1.2 mm. 



Remarks. — P3 is very simple, without anterior basal cuspule and 

 with the sloping heel not forming a distinct cusp. From its roots, 

 P2 was of about the same size as P3. The posterior mental foramen 

 is beneath the anterior root of P4. 



Family PANTOLESTIDAE Cope, 1884 



In his Bridger memoir Matthew (1909) united various genera pre- 

 viously widely scattered in the system and placed them in the Pantoles- 

 tidae. From a study of Pantolestes, a relatively specialized but well- 

 knowQ Middle Eocene genus, he showed that at least the typical 

 members of the family have basic insectivore characters, overlain by 

 peculiar specializations not closely paralleled in recent insectivores. 

 He interpreted the majority of these specializations in Pantolestes as 

 adaptations to aquatic, or to amphibious, life. At that time he pointed 

 out that Palaeosinopa, then laiown from the Lower Eocene, is closely 

 related to Pantolestes and that Pentacodon, of the Middle Paleocene, 

 might tentatively be placed in this family. In 1918 Matthew revised 

 and carefully described the Lower Eocene Palaeosinopa, again 

 emphasizing its close relationship to Pantolestes, and reviewed the 

 evidence for reference of these genera to the Insectivora. This 

 evidence, which still appears to be adequate for such a conclusion is, 

 in briefest possible outline, that the pantolestid dentition is, indeed, 

 more creodontlike than like any recent insectivore but is also closely 

 similar to the unquestionably insectivore leptictids, and that numerous 

 characters of skull and skeleton (especially the astragalus) are quite 

 unlike any known creodonts or other carnivores but do resemble the 

 Leptictidae and other Insectivora. A special point of the evidence 

 (singled out by Sclilosser, see Matthew, 1918, for undue emphasis) 

 is the position of the posterior mental foramen beneath Mi, a character 

 known only in Insectivora, although not characterizing all members 

 of that order. 



Various fragments suggested the presence of this family in the 

 Upper Paleocene (e. g., Simpson, 1927), and Jepsen (1930a) referred a 



