_Characters for ready recognition._--Differs from _M. e. richardsonii_, in that, in both sexes, least width of color of underparts averages about a third, instead of two-fifths, of greatest width of color of upper parts, and in that skulls of males are a fifth or more lighter, basilar length averaging less than 40; from _M. e. cicognanii_, in that hind foot more than 40 in males, averaging 32 versus 30 in females, and in larger skull, depth of skull at plane of molars being 11.4 versus 10.0 in males and 9.1 versus 8.6 in females. _Description._--_Size._--Male: Twelve adult and subadult males from Aitkin, Minnesota, yield average and extreme measurements as follows: Total length, 316 (291-341); length of tail, 87 (70-101); length of hind foot, 43 (40-44). Two adults from Aitkin each weigh 170 grams. Four adult and subadult females from Elk River and Fort Snelling, Minnesota, yield average and extreme measurements as follows: Total length, 249 (240-260); length of tail, 61 (55-65); length of hind foot, 32 (30-33). _Color._--As described in _Mustela erminea richardsonii_ except that, least width of color of underparts averaging, in males from Minnesota, 32 (19-51) per cent of greatest width of color of upper parts. Black tip of tail in 12 male topotypes in white winter pelage averaging 52 (45-58) mm. which is 60 (53-66) per cent of length of tail-vertebrae. _Skull._--Male (based on adults from Aitkin): See measurements and plates 2-4. As described in _Mustela erminea richardsonii_ except that: Weight of 2 adults from Aitkin, 2.2, 2.3 grams (9 subadults from T. 61 N, R. 26 W, average 1.95 grams); basilar length, 39.7 (38.5-40.7); length of tooth-rows rarely more (usually less) than length of tympanic bulla. Female (based on adults from Minnesota as listed in table of cranial measurements, which see): See plates 9-11. As described in _Mustela erminea richardsonii_ except that: Weight, of a subadult from T. 61 N, R. 26 W, 0.91 grams; basilar length, 32.8 (31.8-33.6); breadth of rostrum rarely equal to as much as 30 per cent of basilar length. From _richardsonii_, topotypes of _bangsi_ differ in that cranial measurements in males are approximately 7 (5-9) per cent less, linear measurements of teeth are 10 (9-11) per cent less and the skull is a fifth lighter. In relation to basilar length the tympanic bullae of _bangsi_ are longer. Skulls of females are individually indistinguishable, those of _bangsi_ averaging approximately 1 per cent less in linear measurements. Comparison with the smaller cicognanii is made in the account of that subspecies.