In northwestern Colorado, southern Wyoming, and possibly through the Bear River Divide into southeastern Idaho, long-tailed weasels here referred to nevadensis approach longicauda in large size and occasionally in other features, more closely than do specimens of nevadensis from most other places in its range. This tendency is thought to be significant for much of the area in question lies in or below the Transition Life-zone, the same life zones in which farther to the eastward true longicauda occurs.
One specimen that illustrates this approach to longicauda is an adult male, no. 2334, collection of E. R. Warren, from 6160 feet, Lay, Routt [now Moffat] County, Colorado. In large size and, relative to the basilar length, shorter rostrum and shorter tympanic bullae, it agrees with longicauda but the darker color and, relative to the basilar length, narrowness of the rostrum, interorbital region, zygomatic expanse and the shallowness through the region of the postorbital processes place it with nevadensis. Of two other specimens from Steamboat Springs, Routt County, a young male, no. 4010, in the collection of E. R. Warren, has a hind foot (50 mm.) as long as in longicauda; and the other, no. 138195, U. S. Nat. Mus., an adult male, agrees well enough in size and proportions with nevadensis but has the coloration typical of longicauda.
From Wyoming, one subadult female, no. 177553, U. S. Nat. Mus., from Garrett, is intermediate in size and coloration but is nearer to nevadensis in these particulars, as it is in all other points considered except size of the molar teeth which are as large as in longicauda and larger than in any female nevadensis from Colorado or California. Another female, an adult, no. 179304, U. S. Nat. Mus., from Lonetree, Wyoming, agrees with longicauda in size of skull. Indeed, ten of seventeen cranial measurements exceed the maximum for Colorado-taken nevadensis. Where differences exist in relative proportions of the skull as expressed in percentages of the basilar length, the specimen approaches nevadensis in 5 instances and longicauda in only 3. The color is intermediate but much nearer that of nevadensis with which the animal agrees also in external measurements. Ten subadults (5 of each sex) from within 12 miles of Laramie (not Fort Laramie) show greater resemblance to nevadensis but definitely approach longicauda. Average external measurements are: ♂, 408, 155, 44; ♀, 361, 134, 40. The two other specimens examined from this general locality, a young female, no. 2711, Mus. Vert. Zoöl., from Fort Bridger, and a subadult female, no. 188377, U. S. Nat. Mus., from Bridger Pass, show no departures from nevadensis of similar age.
The specimens from scattered localities in the Transition Life-zone of northwestern Colorado and southern Wyoming are larger than nevadensis is elsewhere, and also in certain other features resemble longicauda of the plains to the eastward. Everything considered, the animals in question are much more like nevadensis than longicauda. Study of more specimens, especially from Wyoming, might provide grounds for recognizing as a different subspecies the animals in this large area comprising parts of Colorado and Wyoming from which so few specimens now are available. Possibly the name Putorius culbertsoni Coues would apply. Decision on that point will require adequate material from the type locality, Fort Laramie. See discussion of this name under M. f. longicauda.
In southeastern Idaho males are larger than they are at most other places within the range of nevadensis. An average of 7 adults and subadults from Pegram, Montpelier, Springfield, and the vicinity of Pocatello, reveals, when compared with the average of nevadensis from Colorado and that of longicauda from the Great Plains, that this population from southeastern Idaho is nearest to longicauda in linear measurements of the orbitonasal length, mastoid breadth, length of tympanic bullae, and as expressed in percentage of the basilar length, length of tooth-row, breadth of rostrum, and zygomatic breadth. In all other points of size, relative proportions and color, the animals approach nearer to, or actually agree with, nevadensis.
The specimens commented upon clearly show intergradation between nevadensis and longicauda. Similarly, the specimens from Scottsbluff County, Nebraska, here referred to M. f. alleni, by their larger size suggest intergradation of that subspecies with the larger nevadensis-longicauda stock although the approach is more toward longicauda than nevadensis. Between oribasus and nevadensis, however, there is no lack of material showing intergradation. As set forth in the account of oribasus, specimens from Montana are truly intermediate structurally as well as geographically.
Intergradation with washingtoni is shown by specimens from the northern part of the Cascade Range in Chelan and Okanogan counties, Washington. The adult male, U. S. Nat. Mus., no. 235183, from Bald Mountain, is referable to washingtoni on the basis of cranial characters but all the other adult and subadult specimens examined from Chelan and Okanogan counties are nearer nevadensis on the basis of cranial characters. Indeed, some show no approach to washingtoni in cranial characters. As might be expected on geographic grounds, the specimen from Easton, U. S. Nat. Mus., male subadult, no. 116870, shows approach to washingtoni. This is true of the coloration of the hind limbs, small size of the tympanic bullae, and relatively greater length of the preorbital part of the skull. However, the greater width of the light color of the underparts and relatively great breadth across the mastoid processes and zygomatic arches are points of agreement with nevadensis. Similarly, a series of 7 specimens from the Entait River, 20 miles above its mouth, in tone of color is nearer to washingtoni, as is one of the two skulls of adult males in length of the preorbital region. However, in greater breadth of the skull otherwise, and in the relatively great width of the light color of the underparts, the animals are nearer to nevadensis, to which they are here referred. Some of these characters mentioned above in which departure is shown from typical nevadensis are characters that show approach to altifrontalis. This is especially true of the more intense coloration and restriction of the color of the underparts.