ARTIFICIAL KEY TO AMERICAN SPECIES OF THE GENUS MUSTELA
A Length of upper tooth-rows less than 20 mm. in males and 17.8 mm. in females.
B Postglenoid length of skull more than 47 per cent of condylobasal length.
C Tail without a black pencil and with at most a few black hairs at extreme tip; in both sexes mastoid breadth ordinarily exceeds breadth of braincase, _Mustela rixosa_, least weasel, p. 168
C' Tail with a black pencil; in females mastoid breadth ordinarily exceeded by breadth of braincase, _Mustela erminea_, ermine, p. 87
B' Postglenoid length of skull less than 47 per cent of condylobasal length.
D Tail with distinct black tip; midventral line white, yellowish, orange, not same color as upper parts; p2 present; thenar pad on forefoot absent, _Mustela frenata_, long-tailed weasel, p. 193
D' Tail without black tip; midventral line same color as upper parts; p2 absent; thenar pad on forefoot present, _Mustela africana_, tropical weasel, p. 406
A' Length of upper tooth-rows more than 20 mm. in males and 17.8 mm. in females.
E Abdomen all white; face with blackish mask; m1 lacking even a trace of a metaconid; distance between upper canines more than width of basioccipital as measured between foramina situated midway along medial sides of tympanic bullae, _Mustela nigripes_, black-footed ferret.
E' Abdomen dark brown, like back; face uniformly brown without blackish mask; m1 with incipient metaconid; distance between upper canines less than width of basioccipital as measured between foramina situated midway along medial sides of tympanic bullae, _Mustela vison_, mink, American mink.
_Diagnosis._--Legs short; body relatively long; adults 190 mm. to 700 mm. in total length; skull ranging in basilar length from 16 to 70 mm.; facial angle slight; tympanic bullae greatly inflated (moderately in _Lutreola_), cancellous, and with paroccipital processes closely appressed to bullae; palate behind upper molars; dental formula:
I 3 C 1 P 2-3 M 1 -, -; -, -; -, ---; -, -; inner moiety of M1 larger than outer; P4 i 3 c 1 p 3-2 m 2
with simple deuterocone; in m1 inner moiety of M1 larger than outer; P4 with simple deuterocone; in m1 trigonid longer than talonid, metaconid absent (incipiently developed in _Lutreola_), and talonid trenchant.