External dimension as a whole decreases from north to south, although not uniformly. For example, the smallest individuals are of the southernmost geographic subspecies (_Z. p. luteus_), but the largest are of the subspecies (_Z. p. utahensis_) that is near the geographic center of the range for the species. In the skull there is geographic variation in the length and shape of the zygomata, size and shape of the incisive foramina, alignment of maxillary tooth-rows, size and shape of auditory bullae, position of the postpalatal notch in relation to M3, and the presence or absence and size of the medial projection on the inferior ramus of the zygomatic process of the maxillary. NATURAL HISTORY _Habitat._--_Zapus princeps_ occurs most commonly adjacent to streams where grasses and herbs are in lush growth. It frequents mountain meadows neighboring small streams and is often taken from alder, aspen, or stands of willow, where the moist ground supports a heavy undergrowth of herbs. Davis (1939:330) found these mice in heavy herbage along a small stream bordered by quaking aspen near Victor, Teton County, Idaho. They were found along streams bordered by willow, rose, alder, huckleberry, sedges, and herbs of various kinds at Alturas Lake, Mill Creek, and at the head of the Pahsimeroi River. Linsdale (1938:195) found jumping mice in the Toyabe Mountains, Nevada, near the streamsides or in seepy areas close to the streams where associated vegetation included rose, willow, wild peach, sage, grasses, and herbs. In the Uinta Mountains, Utah, R. D. Svihla (1931:264) obtained them from willows along streams in mountain parks. Borell and Ellis (1934:37) in the Ruby Mountains, Nevada, found jumping mice to be common in heavy vegetation along streams. Louise Kellogg (1916:369) obtained jumping mice in northern California; all were near water, in grassy meadows, or under alders where vegetation was dense. _Zapus princeps_ is locally abundant, but its numbers seem to vary considerably from year to year as well as seasonally. Early autumn, when young of the year are abroad, seems to be the period of greatest abundance. Moore (1928:154) remarks that runways were plainly marked and well strewn with four-inch pieces of brome-grass. Davis (1939:334) notes that _Z. princeps_ has runways, and found that sections, four inches long, of cut grass piled in runways was good evidence of the presence of the mouse.