_Remarks._--Specimens from Berriozobal, Zacatecas, in the width of the rostrum and the configuration of the nasals, are intermediate between _Dipodomys ordii fuscus_ and _D. o. palmeri_. In color these animals are more nearly like typical representatives of _D. o. palmeri_. The animals from Jalisco and Guanajuato are not typical but the characters differentiating them from _D. o. palmeri_ are merely modifications of such slight degree that they all have been placed with that subspecies. _Specimens examined._--Total, 56, distributed as follows: =Zacatecas=: Canitas, 3 (USBS); Berriozobal, 10 (USBS). =San Luis Potosí=: Potrero Santa Ana, 7.6 mi. S Matehuala, 2 (MVZ); Jesus Maria, 12 (USBS); 2 mi. NW San Luis Potosí, 8 (MVZ). =Aguascalientes=: 1 mi. N Chicalote, 1900 m., 4 (MVZ). =Jalisco=: 9 mi. N Encarnación, 1900 m., 1 (MVZ). =Guanajuato=: Celaya, 8 (USBS). =Hidalgo=: Ixmiquilpan, 1 (USBS); Irolo, 12 (USBS). CONCLUSIONS 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.