From the other subspecies of small-sized weasels of more northwestern occurrence, namely _anguinae_, _fallenda_, _olympica_, _streatori_ and _gulosa_, _muricus_ differs in lighter color of upper parts, wider light-colored underparts and relatively longer black tip of tail.
_Skull._--Male (illustrated by 5 adults in table of measurements, which see): See plate 7. As described in _Mustela erminea richardsonii_ except that: Weight, 0.78 (Wheeler Peak) and 0.85 (Black Butte) grams; basilar length, 30.6 (29.8-31.2); length of tooth-rows more or less than (approximately equal to) length of tympanic bulla.
Female (illustrated by 6 adults in table of measurements, which see): See plates 12-14. As described in _Mustela erminea richardsonii_ except that: Weight, 0.60 (0.575-0.645); basilar length, 28.0 (27.3-29.4); breadth of rostrum approximately 30 per cent of basilar length.
In comparison with _streatori_, males average smaller in every measurement taken with no overlap in most dimensions; 25 per cent lighter; anterior margin of tympanic bulla more nearly flush with squamosal, that is to say less protruded from braincase; in relation to other dimensions of skull, braincase shallower anteriorly (at plane of last molars) and deeper posteriorly (at anterior end of basioccipital). Females average smaller in every measurement taken except mastoid and zygomatic breadths which are actually more; 6 per cent lighter; in relation to other parts of skull, preorbital and interorbital parts slightly smaller; in relation to length of skull, braincase shallower. Comparison with _invicta_ and _gulosa_ is made in the accounts of those subspecies.
_Remarks._--The smallest males of the entire species are of this subspecies and the females of it are barely larger than those of _olympica_ and _gulosa_ and hence are among the three smallest. The material now available consists only of one or a few specimens from each of several widely separated localities. If as many specimens per unit area were available as there are of the species _M. erminea_ from southern British Columbia, geographic variation warranting the division of _muricus_ into more than one subspecies might be revealed. Evidence pointing in this direction is comprised in the pale color and small size of the pair of adults from Wheeler Peak on the eastern border of Nevada; the suggestion is that there is a distinct pale race of small individuals in the isolated spots of boreal life-zone in the mountains of the desert. The color and size of the specimens from the Toyabe Mountains, and that from the Pine Forest Mountains, both places also in Nevada, nevertheless, lend no support to this suggestion. Comparison of specimens from the Rocky Mountains of Colorado with those from the Sierra Nevada of California gives no basis for recognizing more than one subspecies. Therefore, _Putorius streatori leptus_ Merriam with type locality at Silverton, San Juan County, Colorado, falls as a synonym of the earlier named _Putorius (Arctogale) muricus_ Bangs with type locality at Echo, El Dorado County, California. Furthermore, specimens from northern New Mexico, the southernmost known area of occurrence for the subspecies (and for the species), are as large as specimens from far north in the range of the subspecies, say, in northwestern Wyoming; there is therefore no evidence of progressive decrease in size to the southward as in advance of study I supposed existed in _muricus_. This erroneous supposition was held because I knew that there was a decrease in size to the southward in the species as a whole and also in each of the subspecies _richardsonii_ and _invicta_ directly to the north of _muricus_.