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).