Female: Measurements of two subadult female topotypes are as follows: Total length, 346, 318; length of tail, 116, 110; length of hind foot, 39, 35.5. Tail amounts to 50 per cent and 54 per cent of body-length respectively. Length of hind foot more or less than (about equal to) basal length. The average differences in external measurements of the two sexes are: Total length, 111; length of tail, 50; length of hind foot, 12.5. _Externals._--As described in _Mustela frenata noveboracensis_. _Color._--As described in _Mustela frenata noveboracensis_ except that black tail-tip in series of 10 males in full winter pelage 60 (45-80) mm. long; thus averaging 39 per cent of length of tail vertebrae. _Skull and teeth._--Male (based on 3 adults): See measurements and plates 16-18. As described in _Mustela frenata noveboracensis_ except that: Weight, 4.2 (4.1-4.3) grams; basilar length, 45.7 (44.9-46.9); zygomatic breadth more or less than (about equal to) distance between anterior palatine foramen and anterior margin of tympanic bulla; least width of palate rarely less than length of P4; anterior margin of masseteric fossa behind or directly below posterior half of m2. Female (based on 2 subadults): See measurements and plates 31-33. As described in _Mustela frenata noveboracensis_ except that: Weight, 2.0 (1.9-2.1) grams; basilar length, 37.3, 38.2. Comparison of the skull with that of _M. f. noveboracensis_ is made in the account of that subspecies. _Remarks._--Excepting a specimen in the Academy of Natural Sciences of Philadelphia, obtained in 1893, and two in the Boston Society of Natural History, obtained in 1925, I have seen no material of this subspecies in addition to that examined by Bangs at the time he prepared the original description in 1899. Anderson (1945:56, 57) records a specimen, Canadian National Museum Catalogue Number 18426, from Kamouraska County, Quebec, as of this subspecies and thinks that _occisor_ occurs north of Maine "locally to south side of lower St. Lawrence River in Quebec; probably also in western New Brunswick." So far as the available material of occisor permits one to judge, it is distinguished from _noveboracensis_ by a combination of characters no one of which invariably can be relied upon as diagnostic. Employing adult males, average differences indicate that _M. f. occisor_ is larger in each of the external and cranial measurements; tail relatively longer; black tip of tail relatively shorter; mastoid and zygomatic breadth relatively greater and zygomatic arches more nearly square posteriorly.