_Characters of the species_: _External._--Size medium to large (total length 216 mm to 247 mm); tail longer than head and body (129 mm to 148 mm) and bicolored, pale brown to grayish-brown above, white to yellowish-white below; hind feet long (31 mm to 34 mm), grayish-white above; back variable from yellowish-gray to salmon-brown and ochraceous; sides paler than back; lateral line usually present but sometimes indistinct or entirely absent (when present usually clear Ochraceous-Buff); ventral coloration white, usually suffused with ochraceous; ears usually dark, sometimes flecked and usually narrowly edged with light color; guard hairs average 142 microns (130u to 168u) in diameter; underhair with medullary pigment in form of hollow squares; cuticular scales of underhair larger and fewer than in other species. _Baculum._--Size medium (total length 5.6 mm to 6.6 mm); base moderately broad (0.7 mm to 0.8 mm); tip narrow (0.26 mm to 0.31 mm) rounded and dished out in dorsal aspect, blunted; shaft rounded, slightly sinusoidal, recurved at tip. _Skull._--Large, not exceptionally broad and deep in relation to length; rostrum broad but tapering; pterygoid fossa moderately narrow; anterior ramus of zygomatic process usually broad; incisive foramina usually broadly rounded and elongate; auditory bullae usually moderately inflated; coronoid process of mandible relatively short. Upper premolars of medium size (averaging .55 mm in length and .50 mm in breadth), sometimes functional, with occlusal surface normally divided by single shallow re-entrant fold; m1 relatively short, narrow anteriorly. [Illustration: FIG. 46. Distribution of _Zapus princeps_. Guide to subspecies 1. _Z. p. cinereus_ 7. _Z. p. oregonus_ 2. _Z. p. curtatus_ 8. _Z. p. pacificus_ 3. _Z. p. idahoensis_ 9. _Z. p. princeps_ 4. _Z. p. kootenayensis_ 10. _Z. p. saltator_ 5. _Z. p. luteus_ 11. _Z. p. utahensis_ 6. _Z. p. minor_] GEOGRAPHIC VARIATION There are 11 subspecies recognized, most of which are in the mountains of the western United States and southwestern Canada. There is geographic variation in color, relative proportions of external parts (tail, hind feet, head, and body), and shape and size of the skull. Three basic types of coloration occur in _Z. princeps_, as pointed out by Hall (1931:9). Yellow-sided dark-backed jumping mice exemplified by _kootenayensis_, _idahoensis_, and _utahensis_ are found to the eastward in the Rocky Mountains. Reddishbrown-sided, brown-backed jumping mice typified by _luteus_ and _pacificus_ are found to the westward in the Sierra Nevada and in New Mexico and Arizona; mice with yellowish-buff or pinkish-buff-sides and light backs are the subspecies, _cinereus_, _curtatus_, and _oregonus_, that occur in the intervening Great Basin.