Grinnell (1922:95-96) arranged the Recent species of _Dipodomys_ in nine groups. Davis (1942:332) also proposed an arrangement of nine groups in which he combined the Compactus and Ordii groups of Grinnell, established a new Elator group by removing _Dipodomys elator_ from Grinnell's Phillipsii group, and in linear arrangement, Davis shifted the Spectabilis and what remained of the Phillipsii groups to new positions. Burt (1936:152) arranged Grinnell's groups into three (unnamed) groups solely on the basis of the structure of the baculum. In the arrangement proposed by Grinnell, two of his nine groups contained only one species each, one other, the Microps group, has since been shown to contain only one species and another, the Compactus group, contained only kinds which are, by me, regarded only as subspecies of _Dipodomys ordii_. To my mind neither Davis nor Burt added to or fundamentally changed the basic concepts as set forth in 1922 by Grinnell. Owing to the paucity of material at that time, especially from areas of intergradation, Grinnell's groupings and arrangement were as nearly natural as could be expected. With the accumulation of additional material and with the knowledge that certain kinds treated by Grinnell as full species are in actuality subspecies, it is felt that the several species of kangaroo rats can best be arranged in six groups which, from the least to the most specialized, are as follows: ORDII GROUP.--Composed of the subspecies of _Dipodomys ordii_ and _Dipodomys microps_. Grinnell placed these two species in separate groups; Burt on characters of the baculum alone placed _D. microps_ with _Dipodomys deserti_ and _Dipodomys spectabilis_. Within the single species _D. ordii_, I find that the difference in shape and size of the baculum between the subspecies of _D. ordii_ is as great as the difference which Burt (1936:154-155) found between the full species _D. agilis_ and _D. microps_. The characters of the baculum are an aid, but not in and of themselves an adequate basis, for determining the natural relationships of the groups of species. Certainly the remainder of the morphological differences between _D. deserti_ and _D. microps_ are so great that I doubt that the similarity in the baculum is significant, at least in this one instance. The chisel-shaped lower incisors of _D. microps_ appear to be a specialization. They may enable _D. microps_ to utilize more woody types of vegetation than can _D. ordii_. Both species occupy the same territory over much of their geographic range, probably because they eat different kinds of food.