_Characters of the species_: _External._--Size medium to large (total length 221 mm to 238 mm); tail longer than head and body (131 mm to 149 mm) and bicolored, brown above, white to yellowish-white below; hind feet long (31 mm to 34 mm), grayish-white above; back various hues and tones of ochraceous and tawny; sides paler than back; lateral line separating sides from ventral surface usually distinct and bright; ventral coloration white, usually with suffusion of ochraceous; ears usually dark, sometimes flecked, and usually narrowly edged with color of sides; guard hairs average 141 microns (133u to 155u) in diameter; underhair with medullary pigment in narrow, hollow rectangles; cuticular scales of underhair smaller and more numerous than in other species. _Baculum._--Size large (total length 6.7 mm to 7.4 mm); base broad (0.7 mm to 0.9 mm); tip broad (0.44 mm to 0.57 mm); spade-shaped in dorsal aspect and tilted upward, gradually tapering to thin-edged tip; shaft rounded, straight. _Skull._--Large, broad and deep in relation to length; pterygoid fossa broad; anterior ramus of zygomatic process of maxillary relatively narrow; nasofrontal juncture relatively broad; coronoid process of mandible elongate. Upper premolars relatively large (averaging .70 mm in length and .75 mm in width), usually functional, occlusal surface with labial re-entrant fold forming crescentine loop incompletely enclosing single central cusp; m3 relatively large, elongated; m1 elongated, broadly rounded anteriorly. GEOGRAPHIC VARIATION There are four subspecies currently recognized, all of which are confined to the Pacific coastal region of North America (See fig. 45). The features that vary geographically are external size, color of pelage (shade and tone of upper parts and tint of lower parts), and dimensions of certain cranial structures (zygomata, braincase, incisive foramina, palatal bridge, auditory bullae, and pterygoid fossae). External size is smallest in the southernmost geographic race (_Z. t. orarius_) and largest in the northernmost geographic race (_Z. t. trinotatus_). This decrease in size from north to south is clinal and is in keeping with Bergman's Rule which postulates that within one species the smallest individuals occur in the warmer parts of its geographic range. [Illustration: FIG. 45. Map showing distribution of _Zapus trinotatus_. 1. _Z. t. eureka_ 3. _Z. t. orarius_ 2. _Z. t. montanus_ 4. _Z. t. trinotatus_]