Toward the southern part of its range where _noveboracensis_ meets _M. f. olivacea_ there is a marked increase in yellowness of the underparts. This coloration of the underparts, since it is not so well marked in the northern part of the range of _noveboracensis_, might be regarded as showing intergradation with _olivacea_ and _primulina_, each of which has far more intensely colored underparts than does _noveboracensis_. Excepting this increase of yellow on the underparts, however, there are few if any characters of _noveboracensis_ which undergo marked change as approach to the range of _olivacea_ is made. Indeed, the characters of _noveboracensis_ remain constant to within a relatively short distance of the geographic range of _olivacea_.
Notwithstanding the state of affairs described above, intergradation seems to take place. Three specimens referred to _noveboracensis_ but which at the same time are regarded as intergrades with _olivacea_ are as follows: No. 28.300, Charleston Museum, from five miles east of York, South Carolina, is an adult female with a badly crushed skull. In external measurements the specimen agrees with _noveboracensis_. The underparts, as regards color and width, are intermediate. The general proportions of the skull and tympanic bullae agree with those of _noveboracensis_ but the skull is larger than in any female of true _noveboracensis_ and approaches that of _olivacea_. The same can be said of a young female, no. 80, Ohio State Museum, from Roswell, Georgia.
Another female, no. 171559, U. S. Nat. Mus., from Lookout Mountain, 1500 ft., Fort Payne, Alabama, is barely subadult. The external measurements are nearer those of _olivacea_. The color and narrowness of the underparts are typical of _noveboracensis_. The proportions and especially size of the skull show approach to _olivacea_, though they are nearer to _noveboracensis_ when all features are taken into account. In the northern part of its range individuals of _noveboracensis_ attain larger size than farther south. This tendency reaches its extreme, in males at least, in _M. f. occisor_ of Maine. Specimens of _noveboracensis_ from the Adirondacks of New York average larger (see cranial measurements on page 418) than those from farther south, and thus approach _occisor_ in size as well as in geographic position. Also, occasional individuals which strongly show characters of _occisor_ are found even farther south than the Adirondacks of New York. This is true of no. 96518, U. S. Nat. Mus., [M] ad., from Lunenburg; Massachusetts. The animal has a large skull of relatively great width much as in _occisor_, although its external measurements, relative length of tail and long, terminal, black brush place it with _noveboracensis_ rather than with _occisor_. Of a pair of specimens from Ossipee, New Hampshire, the male, no. 77108, U. S. Nat. Mus., has a long (175 mm.) tail, and short (60 mm.) black pencil as in _occisor_, although otherwise it is referable to _noveboracensis_. Still another specimen, a subadult male, no. 4193, Mus. Comp. Zoöl., from Upton, Maine, has a longer (51 mm.) hind foot than _noveboracensis_ although it otherwise agrees with that subspecies. As remarked by Bangs (1899:55), other than fully adult specimens from the range of _occisor_ are "troublesome," and would not be selected as distinct from _noveboracensis_ if placed in a series of that subspecies, say, from New York State. In view of the facts that several specimens from intermediate localities combine the characters of _noveboracensis_ and _occisor_, that _noveboracensis_ in the northern part of its range averages larger than it does farther south and thus approaches _occisor_ in size, and that occasional large specimens resembling _occisor_ in several, but not all, features sometimes crop up in the northern part of the range of _noveboracensis_, it appears that _noveboracensis_ and _occisor_ intergrade. Therefore they are treated as two subspecies of the single species, _Mustela frenata_.