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BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



Diagnosis.— Yer J slightly smaller than M. latidens. P4 relatively 

 smaller, slender, and with paraconid much stronger. Molars rela- 

 tively narrower. Length: width ratio of M2 (of type) 1.24. Para- 

 conids less reduced and trigonids less compressed than in M. latidens, 

 trigonids about equal to talonids in size. 



Discussion. — The difference in size is slight and may not be sig- 

 nificant, but the differences in structure and proportions are marked 

 and sharply distinguish this from M. latidens. The type has another 

 distinction, not cited in the diagnosis, in the fact that the talonid of 

 M2 is definitely narrower than the trigonid, while in M. latidens it is 



of equal width or slightly wider than the 

 trigonid. There are, however, two isolated 

 M2's from Loc. 50 that closely resemble M. 

 angustidens but that have the talonid in 

 one equal to the trigonid and in the other 

 slightly wider. At present it seems more 

 probable that this is a variable character 

 in the species than that these teeth rep- 

 resent a third species. 



Princeton no. 13758 is a left lower jaw 

 with M2-3, from Loc. 73, especially inter- 

 esting because it is from the Fort Union 

 No. 1, where fossils are very rare and be- 

 cause it is, as far as I know, the only fossil 

 mammal ever found in sandstone in this 

 field. Its M2 agrees very closely with the 

 type of M. angustidens except that the talonid is as wide as the trigo- 

 nid. M3 differs from that of M. latidens in the same way as does M2, 

 being relatively narrower, with stronger paraconid and longer 

 trigonid. 



The dimensions of the four specimens mentioned are given in 

 table 45. 



Figure 55. — MimoMcentes angusti- 

 dens, new genus and species, Prince- 

 ton Univ. no. 13758, left lower jaw, a, 

 Crown view; 6, internal view. Twice 

 natural size. 



Table 45. — Individual measurements {in mm) of lower dentition of Mimotricentes 



augustidens 



