1. There are thirty-five recognizable subspecies of the species Dipodomys ordii of which four are herein, for the first time, recognized by name. Three subspecies, D. o. oklahomae, D. o. sennetti and D. o. compactus, previously were regarded by most authors as full species.

2. The species Dipodomys ordii is divisible into six complexes, or groups, of subspecies on both geographic and morphological bases.

3. Dipodomys ordii is the most generalized Recent species of the genus.

4. The extremes of geographic variation in Dipodomys ordii are greater than in any other species of the genus.

5. Color, at least in Dipodomys ordii, does not seem to be correlated with amount of moisture but rather with color of soil.

6. Clinal variation, from north to south, is shown in the decreasing length of the nasals. This decrease in length of nasals and resultant decrease in size of the nasal chamber may be correlated with the decrease of humidity of the environment.

7. Subspeciation has been enhanced by the late Quaternary mountain building which was prevalent over the western United States.

8. In general, the most primitive kinds of Dipodomys ordii occur at the periphery of the range of the species.

9. Natural selection plus geographical and ecological isolation has undoubtedly been operative in speciation and in subspeciation.

10. Dipodomys deserti is found to be the most specialized species in the genus.

11. Six, rather than nine, groups of species are recognized, on the basis of morphology, as comprising the genus Dipodomys.

12. The center of dispersal for the genus Dipodomys appears to have been in the southwestern United States and the adjoining part of Mexico. A secondary center of differentiation is apparent in the low, hot valleys of central California.

13. Parallel development of species is noted between Dipodomys of the parental center and Dipodomys of the isolated valleys of central California; Dipodomys in the California center is the less specialized.

14. The trend of the dipodomyines, as indicated by the fossil record, has been toward a saltatorial specialization with consequent morphological changes.

15. The morphological change in the direction of saltatorial specialization is clearly evident in the compacting and aligning of the viscera as well as in the lengthening of the distal segments of the hind legs and the tail, tufting of the tail, enlargement of the auditory bullae, shortening of the neck and fusion of the cervical vertebrae for stability and other modifications of the skeleton.