_Color._--Winter and summer pelages indistinguishable; upper parts uniform and ranging from Raw Umber to slightly darker (16_n_), and about tones 1 to 3 of Dark Chocolate of Oberthür and Dauthenay, pl. 342; underparts white, in summer rarely with a faint buffy suffusion in pectoral region; color of underparts extends from chin, and often lower lips, posteriorly to inguinal region, distally on posterior sides of forelegs onto antipalmar faces of toes (sometimes interrupted at and above wrist) and on medial sides of hind legs hardly to knee. Least width of color of underparts averaging, in twelve adults from Blaine and Tillamook, 10 (0-47) per cent of greatest width of color of upper parts. Black tip of tail, in same series, averaging 28 (24-33) mm. which is 40 (34-47) per cent of length of tail-vertebrae. _Skull._--Male (based on 12 adults): See measurements and plates 5-7. As described in _Mustela erminea richardsonii_ except that: Weight, 1.1 (1.0-1.2) grams; basilar length, 33.2 (32.5-33.8); length of tooth-rows more or less than (about same as) length of tympanic bulla. Female (based on 7 adults): See measurements and plates 12-14. As described in _Mustela erminea richardsonii_ except that: Weight, 0.64 (0.60-0.67) grams; basilar length, 28.5 (27.6-29.5); breadth of rostrum more than 30 per cent of basilar length. Comparison with _anguinae_, _fallenda_, _olympica_, _gulosa_ and _muricus_ is made in accounts of those subspecies. _Remarks._--This weasel is rare in collections and the best material of it was obtained by Alex Walker in Tillamook County, Oregon, where he resides. The almost ideal series of 30 specimens showed the range of secondary sexual, age, and individual variation expectable in the small ermines of the Pacific Coast of the United States and was the means of allowing satisfactory decision on questions of classification in the related subspecies in which individuals are of comparable size. Intergradation with each of the geographically adjoining subspecies, _olympica_, _fallenda_, _invicta_, _gulosa_ and _muricus_ is shown by specimens examined. With the last mentioned subspecies, intergradation is shown by two specimens from as far south as Siskiyou County, California, assigned to _muricus_.