Annalsof the Carnegie Museum vol. xxvii Specific characters: taken only from the type. The characters of the referred specimen will be noted in the discussion.(1) Differs from M. parvivorus in having the paraconid of M 2 low, relative to the protoconid, and having no cingular cuspule, while in M. parvivorus the paraconid is high, and the cingulum anterior to the protoconid is drawn into a tiny cuspule.( 2 ) Differs from M. medius in that the C, M^, and M^ are almost twice as large in M. medius as in M. gracilis, although the cheek-tooth rows are about of the same length. M^ and M^ of M. medius are sharply cusped, while in M. gracilis the cusps are low, almost extinct. M^ of M. medius has a broad external cingular table at the posteroexternal corner of the tooth, while the cingulum is close to the metacone in M. gracilis.(3) Differs from M. latidens in having the cingulum of M^ close to the metacone, and the cingulum prominent and continuous internally while in M. latidens there is a broad postero-external table and the internal cingulum is low and discontinuous. Also, M^ of M. latidens is almost as broad transversely as is M^, while in M. gracilis, M^ is markedly reduced. M 2 of M. latidens is somewhat larger than M 2 of M. gracilis, and M 3 is twice as large as the corresponding tooth in M. gracilis.(4) The chief differences between M. exiguus and M. gracilis are proportional. Total length of the lower cheek-tooth dentition in M. exiguus is less than ^ that in M. gracilis, while M 2 of M. exiguus is almost as large, and M 3 fully as large, as the corresponding teeth in M. gracilis.(5) The referred specimen of M. sylvestris in the American Museum (A.M.N.H. no. 13071), has the molars approximately half as large as the corresponding teeth in M. gracilis. I have not had the opportunity to make a direct comparison with the type in this case.( 6 ) The type of M. washakius has the broad postero-external cingular table on M\ which occurs also in M. medius and M. latidens, and is not present in M. gracilis. Also, M^ of M. washakius is extremely short antero-posteriorly, and very low-crowned, while in M. gracilis it is longer and higher crowned.(7) In the fragmentary type of Mimocyon longipes Peterson, sometimes referred to Miacis, each bone is almost exactly twice as large as the corresponding bone in M. gracilis. In the absence of good characters for comparison in Mimocyon longipes, this seems a valid basis for a specific distinction between the two.( 8 ) The type of Prodaphoenus (?) rohustus Peterson, referred by Hay^ to Miacis, is twice as large as M. gracilis, and has a heavy mandible with a distinct chin, while in M. gracilis the mandible is slender and tapering.