NO. I LOWER EOCENE MAMMALIAN FAUNAS — GAZIN 21 



SYSTEMATIC DESCRIPTION OF THE MAMMALIA 



MARSUPIALIA 



DIDELPHIDAE 

 PERATHERIUM EDWARDI Gazin 

 No additional material belonging to the larger and more certainly 

 didelphid of the two species in the La Barge fauna referred to 

 Peratherimn has been encountered in the Knight. The maxilla and 

 jaw representing this species were both found at the Muddy Creek 

 locality about 12 miles north of Big Piney. 



PERATHERIUM CHESTERI Gazin 

 The type lower jaw of this very small species, together with the 

 material of the foregoing form, was discovered at the Muddy Creek 

 locality north of Big Piney. There is, however, a minute jaw in the 

 Princeton University collections (No. 161 16) from the Cathedral 

 Bluffs member of the Wasatch to the north of Flat Top Mountain 

 in the Washakie Basin that may be marsupial and possibly repre- 

 sents this species. Morris (1954), in his study of the Cathedral 

 Bluffs fauna, has listed it as Nyctitherium, sp. The preserved tooth, 

 as in the La Barge jaw, is the penultimate molar. It is a little smaller 

 and relatively shorter than in the type of P. chesteri, but exhibits a 

 similar posterointernally directed hypoconulid, much more lingual in 

 position than in materials that have been referred to Nyctitherium, 

 rather more as in marsupials. Confusion as to the identity of the 

 form represented may lie in the appearance of the entoconid, which 

 seems very small and close to the hypoconulid, suggestive in this way 

 of the South American monodelphids although the paraconid and 

 anteroexternal cingulum are not so importantly extended. Examina- 

 tion under X 48 magnification, however, reveals an irregularly pitted 

 area forward from the hypoconulid, indicating that the greater part 

 of the entoconid may be missing through damage, possibly modified 

 by wear. A better forward development of the entoconid strengthens 

 the suggestion of didelphid affinity. The lower jaw is a little less 

 robust and of slightly less depth than in the type of P. chesteri. 

 Neither are sufficiently preserved posteriorward to show the nature 

 of the angle. 



PERATHERIUM MORRISI, new species 

 (Plate I, figure i) 

 Type. — Right ramus of mandible with two molars, P.U. No. 161 15. 

 Horizon and locality. — Cathedral Bluffs member, north of Flat Top 

 Mountain, sec. 4, T. 115 N., R. 93 W., Washakie Basin, Wyoming. 



