1953 preblei (Zapus hudsonius) described as new on [page 452] of this paper.
External parts.—The total length, the length of the tail, and the length of the hind foot are useful to some extent in distinguishing species and subspecies. Geographic variation in these measurements is clinal in some species. For example, Zapus trinotatus, which inhabits the western coast of North America, decreases in size from the northern to the southern part of its range. There is considerable overlap in external measurements, in specimens of the same age, between the species Z. trinotatus and Z. princeps, but only slight overlap between Z. princeps and Z. hudsonius and between Z. trinotatus and Z. hudsonius. If all collectors measured external parts in the same way the measurements would be more useful for differentiating one species from another.
Pelage.—The pelage, both in its entirety and as individual hairs, provides taxonomic characters as has been pointed out by Moojen (1948:324) for the genus Proechimys, by Williams (1938:239) for the Insectivora, and by Hausman (1920:496) for several groups of mammals. In addition to the sensory hairs, facial vibrissae, nasal hairs, and carpal vibrissae, there are three kinds of hairs in the normal coat of Zapus: guard hairs, overhairs, and underfur. The guard hairs and underfur differ in different species (see [figs. 35-37]).
The guard hairs taper at both ends, are elliptical in cross section, and are wider and longer than the other two kinds of hair. The bases of the guard hairs are grayish, and the amount of pigment gradually increases distally to a dark brownish or blackish shade. The guard hairs vary in greatest diameter from 96 microns to 168 microns, depending upon the species, and variation in diameter provides characters of taxonomic worth. No clinal variation in diameter of the guard hairs was detected. In Z. hudsonius the guard hairs average 115 microns (96-140) and are significantly narrower than those of Z. princeps and Z. trinotatus, which average 142 microns (130-168) and 141 microns (133-154), respectively. Pigmentation of the guard hairs contributes little information useful in separating the species of Zapus. All of the species have a prominent compounded medulla in which the pigment cells anastomose to form a labyrinthine column.
The individual hair of the underfur is cylindrical and tapers abruptly at each end; it is short, thin, flexible, and usually is bicolored on the back and sides of the mouse. The apical zone is yellow-brown (for example, Ochraceous-Buff) and the proximal part is whitish or grayish, which gradually darkens to near black subapically.
The width of a hair in the underfur is of no taxonomic significance, in that individual variation exceeds that between species.
The pattern of the pigment in the medulla of the hair, however, does vary specifically. Comparable samples from Z. trinotatus, Z. princeps, and Z. hudsonius of the same age, sex, and season reveal a pattern characteristic for each species (see [figs. 35-37]).
All species of Zapus agree closely in color pattern. A broad longitudinal dorsal band of some shade of yellow-brown flecked with black hairs is bordered by a lateral band of a lighter color usually containing fewer black hairs than on the dorsum. The underparts are usually white but are sometimes suffused with color resembling that on the sides. Between the white underparts and the darker color of the sides there is often a narrow, clear ochraceous stripe. Dorsal and lateral hairs are uniformly grayish-white at their bases; only the distal parts of the hairs are responsible for the external color of the animal.