Long tail and long hind foot would seem to be specializations for saltation and the two would be expected to be correlated. Actually there is no significant correlation in _D. ordii_. _D. o. celeripes_, in which the hind foot is near the mean for the species (39.8 as opposed to the mean of 40.7), has the shortest tail. _D. o. compactus_ has a short tail (117.0 mm.) but a medium-sized hind foot. _D. o. nexilis_ and _D. o. terrosus_ have both a long hind foot and long tail. Cranial measurements vary less, probably because one person can measure a series with a uniformly subjective error. External measurements, however, are liable to a greater degree of subjective error. The total length of the skull varies from 35.4 mm. in _D. o. attenuatus_ to 41.3 mm. in _D. o. terrosus_. In no one series of adults from one locality, however, is the variation so marked as it is for the species as a whole. The usual range of variation in length of skull in any given series is not, as a rule, more than 2.5 mm. Cranial indices (breadth across bullae/length of skull × 100) as established for random samples of the different species of the genus (exclusive of _D. ordii_) ranged from 60.8 to 67.6. In the subspecies of _D. ordii_ the same index varies from 59.7 to 65.2 with an average of 63.4. In other words, the degree of specialization indicated by this one index, in a few subspecies of _D. ordii_, is almost as great as that in _Dipodomys deserti_, which on the basis of total morphology appears to be the most specialized species in the genus. Also, on the basis of this same index, some subspecies of _D. ordii_ are more generalized than is any other species in the genus. There is a general tendency for the nasals to decrease in length and the rostrum to decrease in width as the southern limits of the range of _D. ordii_ are approached. In ascertaining the decrease in length of the nasals an index was obtained as follows: nasals/interorbital width × 100 (see Table 4). The width of the rostrum, however, does not decrease in the same degree, nor at the same rate, as does the length of the nasals. This decrease in length of the nasals and in width of the rostrum may be correlated with the mean annual relative humidity of the environment. It is known (Howell and Gersh, 1936:8) that desert rodents, more exactly kangaroo rats, have a water retention mechanism in the kidneys and walls of the urinary bladder which enables them more efficiently to conserve metabolic water. The significance of the decrease of the area of the nasal mucosa, which seems to be related to relative aridity, is not yet properly understood. In no cranial feature other than shortened nasals and narrowed rostrum, does _Dipodomys ordii_ show a gradation such that it might be termed a cline. Other parts of the skull that were measured do not vary greatly.