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: [M], 408, 155, 44; [F], 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.