Differences from males of _olympica_ are: size larger with no overlap in most measurements; 50 per cent heavier; tympanic bullae longer than upper tooth-rows rather than of about equal length; in relation to basilar length, rostrum shorter, braincase wider and deeper, zygomata more expanded. Females are larger with no overlap in most measurements; 35 per cent heavier; in relation to basilar length, pre-and interorbital regions narrower, braincase deeper and wider across mastoids.
Differences from _streatori_, in males, are: skull averaging larger in every cranial and dental measurement taken; 36 per cent heavier; tympanic bulla longer than, instead of about same length as, upper tooth-rows. In females the inner lobe of M1 is shorter anteroposteriorly; otherwise all measurements of _fallenda_ average larger and it is 33 per cent heavier; rostrum and interorbital region broader in relation to remainder of skull.
In comparison with _gulosa_, skulls of males differ as follows: averaging larger in every measurement taken with no overlap in several dimensions; 50 per cent heavier; tympanic bullae with anterior margins projecting slightly below squamosals rather than flush with same; length of bulla more than, rather than about same as, that of upper tooth-rows. Considering the great difference in size, the relative proportions are remarkably alike. In females, length of inner lobe of M1 about the same; otherwise averaging larger in every measurement taken; 44 per cent lighter; relative to basilar length, tooth-rows longer, skull wider across zygomata and mastoids, rostrum and interorbital regions slightly narrower, skull shallower in plane of last upper molars.
Comparisons with _haidarum_, _invicta_ and _anguinae_ are made in accounts of those subspecies.
_Remarks._--Until the name _fallenda_ was proposed in the course of the present study, most of the specimens of this race were assigned to _streatori_.
Intergradation with _streatori_ is complete as it is also with _invicta_ and _richardsonii_, in other words with each of the subspecies whose ranges meet that of _fallenda_. In color and in size the difference is least between _streatori_ and _fallenda_. As between _fallenda_ and _invicta_ the size is not greatly different and the intergradation in color is gradual. Between _fallenda_ and _richardsonii_ intergradation is somewhat different and to fully appreciate its nature we should remember that the color of _fallenda_ resembles that of the saturate coastal races, _streatori_, _anguinae_ and _olympica_ although the black tip of the tail is longer. In this latter feature and in several cranial details, as well as in greater degree of secondary sexual variation in size, _fallenda_ resembles _richardsonii_. Because the two differ more than do most subspecies of ermine whose ranges meet, some of the intergrades at first inspection appear to be widely different from either parent stock. For example, specimens from Alta Lake, British Columbia, may give this impression because the combination of large size and dark color suggests a kind of ermine different from either _fallenda_ or _richardsonii_. In no instance, however, has there been found in these intergrades any character other than those occurring in one or the other of the two parent races.