The large size of males of spadix, as exemplified by specimens from Elk River (see measurements on page [421]), seems to be retained across the northern part of the range here assigned to the subspecies. This larger size than is found in longicauda from Alberta, is shown also by some specimens from eastern North Dakota which are assigned to longicauda. However, the average of these Dakotan specimens, all characters considered, is nearer to my concept of longicauda.
Inspection of the cranial measurements of spadix shows also that in addition to its large size it is distinguishable from any one of the geographically adjoining races by its relatively (to basilar length) greater, as well as actually greater, mastoidal breadth. This might be included with size as a unique character distinguishing spadix from longicauda and noveboracensis. However, it is not clear whether or not this greater mastoidal breadth is more than a function of the large size.
Excepting the greater mastoidal breadth and generally larger size of the skull, the cranial features distinguishing males of spadix from longicauda are features in which spadix shows approach to noveboracensis. This is true, in spadix, of the relatively longer (in comparison with longicauda) rostrum, relatively lesser zygomatic breadth, relatively shallower braincase measured at the anterior end of the basioccipital, and relatively deeper skull as measured at the posterior borders of the last upper molars. This same approach to noveboracensis already has been pointed out with respect to color of the upper parts and is evident also in the relative shortness of the tail which averages only 51 per cent of the length of the head and body rather than 55 per cent as in longicauda.
Because the longicauda type of animal previously has been regarded as specifically distinct from the noveboracensis type of animal, comment is offered below on selected specimens, referred to spadix, which are regarded as intergrades with noveboracensis or with other subspecies.
No. 8722, Univ. Wisconsin, adult male, in the white winter coat, from north central Itasca County, Minnesota, obviously has characters of M. f. spadix or longicauda that occur to the west and M. f. noveboracensis of the east. Selected outstanding characters of longicauda are its long tail, anteriorly truncate tympanic bullae and large teeth. Characters indicating its affinities with noveboracensis are smaller size of skull, general narrowness of skull, and relatively low tympanic bullae. The skull is intermediate as regards several individual structural features. For example, although long and narrow and in this feature more nearly approaching noveboracensis, the skull is wider than usual in that subspecies and thus approaches that of longicauda or spadix. The hind foot, in the dried state, measures 47 millimeters. This large hind foot, obviously long tail (the specimen lacks external measurements), and anteriorly truncate bullae constitute basis for here referring the specimen to spadix. However, the seemingly small size of the body and the narrow skull clearly show relationship to noveboracensis.
Specimens, referred to spadix, from northern Iowa, are instructive as showing what happens where the ranges of noveboracensis, primulina, spadix, and perhaps longicauda, meet. No. 47167, Univ. Mich. Mus. Zoöl., a nearly adult female, taken on November 22, 1915, at Island, Clay County, and in process of assuming a brown winter pelage, retains enough of the dark summer pelage to show that the color was slightly lighter than average for spadix. The color pattern, white lips, and extension of light color of the underparts onto the feet, agrees with spadix or longicauda as does also the long tooth-row. The overall length of the skull is intermediate between that of spadix and primulina. The proportions of the anterior part of the skull and of the tympanic bullae resemble those found in primulina. A subadult male skull only, no. 123846, American Museum of Natural History, from Webb, Clay County, shows approach to primulina in the narrowness of the rostrum.
A young male from Ruthven, Iowa, no. 48340, Univ. Michigan, has a large skull approaching in size that of spadix, has the longicauda-spadix type of light-colored underparts and color pattern, and is slightly darker above than true longicauda. Another subadult male in the white winter coat from Palo Alto County, no. 35756, Univ. Michigan, has a large skull, which shows approach to primulina in its narrowness anteriorly and in some other features. Although the tail is of moderate length, the body is large as in spadix or longicauda, and the length of the hind foot suggests spadix or longicauda.
A subadult male, no. 425a, Iowa State College, from Manson, Iowa, in brown winter pelage, agrees with primulina in the restriction of the area of the light color of the underparts and in less expanded zygomatic arches. The teeth are intermediate in size between those of noveboracensis and primulina on the one hand and those of spadix and longicauda on the other. In other respects it agrees with, or is more nearly like, spadix.