_Externals._--Longest facial vibrissae dark brown or white and extending beyond ear; carpal vibrissae same color as underparts and extending to apical pad of fifth digit; hairiness of foot-soles (in summer pelage) as shown in figure 20. _Color._--Winter pelage unknown; probably white except, of course, tip of tail. Summer pelage as described in _Mustela frenata longicauda_ except that: Least width of color of underparts averaging, in 3 males from Black Hills, 54 (38-62) per cent of greatest width of color of upper parts. Black tip of tail averaging 43 (40-45) mm. long. Thus, averaging approximately same length as hind foot and in type specimen amounting to 33 per cent of length of tail-vertebrae. _Skull and teeth._--Male (based on the type and no. 7440 Amer. Mus. Nat. Hist., from Hill City, S. Dak.): See measurements and plates 18-20. As described in _Mustela frenata longicauda_ except that: Weight, 3.1 (3.0-3.2) grams; basilar length, 41.0 (40.9-41.0); mastoid breadth not less than postpalatal length; breadth of rostrum more than length of P4; anterior margin of tympanic bulla as far posterior to foramen ovale as width of 4 to 5 upper incisors; height of tympanic bulla more or less than distance from its anterior margin to foramen ovale. Female (based on no. 7441, American Mus. Nat. Hist., from Black Hills, S. Dak.): See measurements and plates 31-33. As described in _Mustela frenata longicauda_ except that: Weight, 2.0 grams; basilar length 37.6. The skull of the female is 35 per cent lighter than the average for the two males. Comparison with _M. f. longicauda_ and _M. f. nevadensis_ reveals that the tympanic bullae average more nearly flat and that the skull is smaller. _Remarks._--Animals of this subspecies were described and named by Merriam in 1896 as a distinct species on the basis of two or possibly three specimens from the Black Hills of South Dakota and the name seems never to have been applied to specimens from other regions. Vernon Bailey obtained only the one specimen, the type, on his trip in 1888, but two more were obtained for the American Museum of Natural History by Walter Granger in 1894.