NO. I LOWER EOCENE MAMMALIAN FAUNAS — GAZIN 5 1 



MESONYCHIDAE 



HAPALODECTES COMPRESSUS Matthew 



(Plate 8, figure 6) 



Portions of both rami of a mandible of the rare mesonychid Hapalo- 

 dectes compressus were found in the New Fork beds on Alkali Creek 

 (Sublette County). The teeth of this specimen, U.S.N.M. No. 22447, 

 are somewhat damaged but a sufficient portion of each remains to 

 demonstrate their exceedingly slender character. Comparison with 

 the type of H. compressus (A.M. No. 12781) from the Lysite shows 

 that the New Fork specimen is comparable in length of teeth and in 

 the depth and slenderness of the jaw, and would not be confused with 

 heavier-jawed Hapalodectes leptognathus. The teeth, if anything, 

 are more slender than in the H. compressus type, or even than in the 

 Lost Cabin jaw (A.M. No. 14748) that Matthew noted as having more 

 slender teeth and a shallower jaw than the type. 



PACHYAENA OSSIFRAGA Cope 



A number of broken pieces of skull and associated jaw and tooth 

 portions (U.S.N.M. No. 22449) from the Bitter Creek locality are 

 referred to Cope's New Mexican species Pachyaena ossifraga. Among 

 these is the greater part of an upper molar, lacking only the deutero- 

 cone, that corresponds remarkably well in size and various other 

 details with Cope's type (U.S.N.M. No. 1096), presumably an M^. 



PACHYAENA GRACILIS Matthew 



About 15 associated teeth (U.S.N.M. No. 22448) of Pachyaena 

 were found in the Gray Bull equivalent of the Knight near the base 

 of the escarpment west of Elk Mountain in the Fossil Basin. These 

 compare very closely in size with various teeth in the type of 

 Pachyaena gracilis (A.M. No. 15728) from the Gray Bull beds in the 

 Bighorn Basin. Their size, nevertheless, is but little less than in the 

 P. ossifraga material. An upper molar, possibly M\ in addition to 

 smaller size, shows somewhat less development of the metacone than 

 in the type of P. ossifraga. 



An incomplete lower cheek tooth from low on the slope of Fossil 

 Butte may also represent P. gracilis. It is, however, rather small and 

 somewhat suggestive of earlier Dissacus. 



PACHYAENA?, sp. 



No additional material believed to represent Pachyaena, other than 

 the two specimens reported in 1952, has been found in the New Fork 



