_Color._--Winter pelage all white except tip of tail. In southern part of range sometimes assumes a brown winter coat. Summer pelage with upper parts ranging from near (16´) Cinnamon Brown to Vandyke Brown. Chin and upper lips white. Remainder of underparts ranging from near (a) Olive Ocher to Ochraceous Buff and Pale Orange Yellow. Tip of tail at all times black. Upper parts of uniform color except for occasional slight darkening of nose. Color of underparts extends distally on posterior sides of forelegs over toes onto antipalmar faces of feet and ankles, on medial sides of hind limbs to ankle, over antiplantar faces of toes and distomedial fourth of each tarsus, and over proximal fifth to third of under side of tail. Least width of color of underparts averaging (in 3 specimens from Elk River) 54 (47-59) per cent of greatest width of color of upper parts. Black tip of tail averaging same length as hind foot and 28 per cent of length of tail-vertebrae. Save for the greater width of the light-colored underparts and relatively short black tip of the tail, both features of _M. f. longicauda_, _spadix_ is variously intermediate, depending on locality, as between _noveboracensis_ and _longicauda_. _Skull and teeth._--Male (based on 3 adults from Elk River, Minn.): See measurements and plates 16-18. As described in _Mustela frenata longicauda_ except that: Weight, 5.6 (5.0-6.5); basilar length, 49.0 (48.7-49.2); zygomatic breadth sometimes less than distance between anterior palatine foramen and anterior margin of tympanic bulla; postorbital breadth more or less (about equal to) width of basioccipital measured from medial margin of one foramen lacerum posterior to its opposite; interorbital breadth more or less than distance between foramen opticum and anterior margin of tympanic bulla; 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; length of tympanic bulla less than length of rostrum; anterior margin of masseteric fossa below talonid of m1. Female (based on 4 adults from Elk River, Minn.): See measurements and plates 31-33. As described in _Mustela frenata longicauda_ except that: Weight, 3.5 (3.3-4.0) grams; basilar length, 42.9 (42.3-43.2); least width of palate more or less than greatest length of P4; tympanic bulla as far posterior to foramen ovale as width of 3 to 5 upper incisors. The skull of the female averages 33 per cent lighter than that of the male.