Female (based on the type, specimen): See measurements and plates 31-33, 40. As described in _Mustela frenata longicauda_ except that: Weight, 3.5 grams; basilar length, 41.6 mm.; zygomatic breadth more than distance between anterior palatine foramen and anterior margin of tympanic bulla; postorbital breadth more than width of basioccipital measured from medial margin of one foramen lacerum posterior to its opposite; least width of palate more than outside length of P^{4}; tympanic bulla as far posterior to foramen ovale as width of 4-1/2 upper incisors; height of tympanic bulla less than distance from anterior margin of tympanic bulla to foramen ovale; length of tympanic bulla less than length of rostrum. If more than one skull were available of the female of _oribasus_ it is believed that the description would agree with that of _longicauda_ in nearly all features. The skull of the female is 30 per cent lighter than that of the average male. Comparison with _longicauda_ reveals that, on the average, skulls of males are larger, relative to the basilar length broader across the mastoids, shallower through the braincase as measured at the anterior end of the basioccipital exclusive of the sagittal crest, with longer rostrum. Compared with _nevadensis_, the skull averages larger in all measurements taken, and has a relatively broader rostrum, relatively greater mastoid breadth and a braincase which is shallower relative to the basilar length. By weight, the skull of _nevadensis_ is a fourth lighter, and in linear measurements 5 to 18 per cent smaller. _Remarks._--Some of the specimens from Montana, which here are referred to _oribasus_, more than half a century ago were listed by Coues (1877:138) under the name _longicauda_. It was not until 1899 that this race was given a name by Bangs, who at that time (1899B:81) accurately made out the distinctive color features. Distinctive cranial characters cannot be described with assurance even now because there still are too few specimens. The type specimen, at one time examined by the present writer, has on the stuffed skin no well-developed mammae, scrotal pouch, or other visible sexual part. Probably the collector's sex mark for female is correct.