There are only three groups of mammals in the Southwest having external cheek pouches.  These are (a) the pocket gophers (Geomyidae), which have strong fore feet, relatively weak hind feet, and short tail, as compared with weak fore feet, relatively strong hind feet, and long tail in the other two; (b) the pocket mice (Perognathus), which are considerably smaller than the kangaroo rats and lack the conspicuous white hip stripe possessed by all the latter; and (c) the kangaroo rats (Dipodomys).

[Illustration:  FIG. 1.—­Range, east of the Colorado River, of Dipodomys spectabilis spectabilis compared with that of Dipodomys merriami.  Cross hatching indicates area of overlapping of the two forms.  The range of Dipodomys deserti, not shown on the map, is west of that of spectabilis, and so far as known the two do not overlap.]

Dipodomys spectabilis spectabilis Merriam requires comparison with three other forms of kangaroo rats in the same general region, namely, D. deserti Stephens, of approximately the same size, and D. merriami Mearns and D. ordii Woodhouse, the last two of decidedly smaller size.  The range of deserti lies principally to the west of that of spectabilis, and the two do not, so far as known, overlap.  On the other hand, merriami and ordii, and subspecies, occur over a large part of the range of spectabilis, living in very close proximity to its burrows; merriami is even suspected of pillaging the stores of spectabilis.  The range of merriami, however, is much more extensive than that of spectabilis (Fig. 1), which argues against a definite ecological dependence or relationship.  Separation of the four forms mentioned may be easily accomplished by the following key: 

Key to Species of Dipodomys in Arizona.

a^1.  Size much larger (hind foot and greatest length of skull more than 42 millimeters); tail tipped with white.

b^1.  Upper parts dark brownish buffy; tail dark brownish or blackish with more sharply contrasted white tip; interparietal broader, distinctly separating mastoids (range in Arizona mainly southeastern part) =Dipodomys spectabilis.=

b^2.  Upper parts light ochraceous-buffy; tail pale brownish with less sharply contrasted white tip; interparietal narrower, reduced to mere spicule between mastoids (range in Arizona mainly southwestern part) =Dipodomys deserti.=

a^2.  Size much smaller (hind foot and greatest length of skull less than 42 millimeters); tail not tipped with white.

     b^1.  Hind foot with four toes =Dipodomys merriami.=

     b^2.  Hind foot with five toes =Dipodomys ordii.=