_Jaculus hudsonius_, Allen, Bull. Essex Inst., 6:61, April, 1874 (part--the part in Carbon County, Wyoming). _Zapus hudsonius_, Merriam, N. Amer. Fauna, 5:72-73, July 30, 1891. _Zapus princeps princeps_, Preble, N. Amer. Fauna, 15:22-23, August 8, 1899 (part). _Type._--Male, adult, skin and skull; No. 54845, Mus. Vert. Zool.; 5 mi. E Warm Lake, 7000 ft., Valley County, Idaho; obtained on July 9, 1932, by Robert T. Orr; original No. 660. _Range._--From Banff, Alberta, southward through extreme southwestern Alberta and extreme southwestern British Columbia, most of the panhandle of Idaho, Kamiak Butte in eastern Washington, western Montana, and western Wyoming (Green, Wind River and Absoroka ranges of the Rocky Mt's). See fig. 46. _Description._--Size, medium; back from near Clay Color to near Warm Buff, usually overlaid with black hairs forming broad dorsal band; sides lighter than back; lateral line indistinct or wanting; belly pure white, occasionally faintly tinged with Ochraceous-Buff; tail indistinctly bicolored, tan to grayish-white below and pale brown above; hind feet grayish-white above; ears dark, edged with white or yellowish-white; postpalatal notch anterior to posterior border of last molars; proximal part of inferior ramus of zygomatic process of maxillary relatively narrow and usually without enlarged median projection; auditory bullae well inflated; incisive foramina relatively narrow. _Comparisons._--From _Zapus princeps kootenayensis_, _Z. p. idahoensis_ differs as follows: Size averaging larger; upper parts with greater suffusion of ochraceous, not grayish or dusty; skull larger; incisive foramina longer and relatively wider; zygomatic breadth averaging greater; nasals broader at tips; auditory bullae more inflated. From _Zapus princeps oregonus_, _Z. p. idahoensis_ differs in: Size averaging smaller; upper parts generally more suffused with black hairs, on the average more yellowish with less ochraceous; skull smaller; incisive foramina narrower (breadth less, instead of more, than 52 per cent of length); palatal bridge shorter; zygomatic arch shorter; pterygoid fossae narrower. From _Zapus princeps utahensis_, _Z. p. idahoensis_ can be distinguished by: Size less; color slightly darker; skull averaging smaller in zygomatic breadth, least interorbital constriction, and occipitonasal length; palate narrower; upper tooth-rows nearly parallel as opposed to diverging anteriorly.