_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.=

On account of the small size, _merriami_ and _ordii_ do not require
detailed color comparison with the other two. The general color of the
upperparts of _spectabilis_ is much darker than that of _deserti_;
whereas _spectabilis_ is ochraceous-buff or light ochraceous-buff
grizzled with blackish, _deserti_ is near pale ochraceous-buff and lacks
the blackish.

The color of the upperparts alone amply suffices to distinguish
_spectabilis_ and _deserti_; but the different coloration of the tail is
the most obvious diagnostic feature. The near black of the middle