536 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 
fuscous to a greater or less extent and intensity. Some specimens, with the 
least dusky, are very light colored—a _ pale yellowish-cinnamon; others 
approach mouse-color, but even in the darkest specimens the decided fulvous 
shade appears at least upon the sides. All this colored portion is plumbeous 
beneath, excepting a little space along the middle of the sides, where basaily 
white hairs have the tawny tips. All the under surface of the animal is snow- 
white to the roots of the hairs. The line of white begins on the side of the 
muzzle and runs along the side of the head, including the pouch; the entire 
fore limb is white; the stripe rises a little on the side of the belly, and thence 
runs along the middle of the outside of the hind limb from the knee to the 
heel, sending a sharp white stripe from the knee across the haunches to the 
root of the tail. The hind foot is white, with a dusky stripe along the sole. 
The whiskers are partly black, partly colorless; their conjoined bases make a 
conspicous black spot on each side of the muzzle. There is some whitish- 
ness in most cases—sometimes altogether wanting—about the eye, and a white 
patch just back of the ear. The front of the ear is sometimes light. The 
tail is dusky-slaty, or sooty-brown, or even blackish, with a broad, firm, white 
stripe on each side from base to near the tip. At the extreme base, the white 
usually encircles the tail; at the other end, the color of the tuft is altogether 
indeterminate; sometimes the white lateral stripes give out before reaching 
the end, leaving the tip entirely dark; sometimes the white extends to the 
very end of the brush, cutting off the dark altogether; and, moreover, the 
white may encroach upon the under side, eutting off the dark from more than 
half the tail; oftener, the brush is mixed dusky and white. Thus the tail 
may end either white or dark, or a mixture of both. It is as variable in this 
respect as the tail of a skunk. The eyes are lustrous black; the nose-pad 
and palms flesh-colored; the claws pale. 
In old museum specimens, long exposed to the light, the above descrip- 
tion may not be verifiable as regards any of the darker markings and shades 
mentioned ; for all the colored portions of the fur finally fade to a dull, pale 
brownish-yellow, or even dingy yellowish-white. Under such circumstances, 
even the rich purplish-chestnut of a mink, for example, ends in dingy whitish. 
Discussion of the species of Dipodomys—Having thus fully exposed the 
characters of the animals of this genus, it remains to consider the mode in 
which, and extent to which, the genus has become differentiated into recog- 
nizable forms, if there be more than one. Various species have been pro- 
