Skulls of adult males of _spadix_ from Elk River, Minnesota, as compared with those of _longicauda_ from Alberta, are larger in every part measured. Relative to the basilar length these skulls of _spadix_ are broader across the mastoid region, narrower across the zygomata, deeper through the plane of the postorbital processes, shallower through the braincase and have relatively shorter tympanic bullae. Whereas the tympanic bullae of _longicauda_ are, on the average, approximately as long as the rostrum (orbitonasal length), in _spadix_ the rostrum is longer than the bulla. Viewed posteriorly, the braincase of _spadix_ is seen to be much shallower and wider than that of _longicauda_. Indeed, the depth of the braincase, measured at the anterior end of the basioccipital, amounts to only 56 per cent of the mastoid breadth in _spadix_ as against 61 per cent in _longicauda_. The longer, waistlike, postorbital constriction, relatively smaller braincase, and especially the relatively narrower zygomatic expanse in _spadix_ imparts to its skull a more slender appearance than has the skull of _longicauda_. These differences are not shown by the skulls of females. To be sure, _spadix_, in most of its cranial measurements, averages slightly larger, has a relatively shallower braincase and is relatively deeper through the postorbital processes, but these differences are so slight that inclusion of one more specimen, of slightly different proportions, in the average might cause the average measurements to read as they do in _longicauda_. Compared with _noveboracensis_, from Massachusetts, adult skulls of _spadix_, taking sex into account, are larger in every part measured and are relatively as well as actually wider and deeper throughout. Also, in _spadix_: Sagittal and lambdoidal crests higher, especially in females; anterior margin of tympanic bulla projecting up sharply from squamosal; occiput more flattened in posterior view; tooth-rows relatively and actually longer but orbitonasal length relatively shorter; postorbital processes more robust; zygomatic arches widely bowed outward rather than evenly rounded; canines larger; squamosal less swollen ventrally, especially in females. Between _noveboracensis_ and _spadix_, the differential cranial characters are greater in number and degree between females than between males. Comparison of the skull with that of _M. f. primulina_ is made in discussion of that subspecies. _Remarks._--Edgar A. Mearns in 1889 and the early nineties took several specimens of this weasel and it was principally on these that Bangs in 1896 (p. 8) based his description. The best material, however, is that from Elk River, Minnesota, collected in later years by Bernard Bailey, and supplemented by one specimen taken in 1885 by Vernon Bailey and another by his sister Anna Bailey in 1891 at the same place.