Wood (1935:78) suggested that _Dipodomys gidleyi_ gave rise to _Dipodomys spectabilis_ and _Dipodomys ordii_, and _Dipodomys minor_ gave rise to _Dipodomys compactus_. However, my own study indicates that _Dipodomys compactus_ is conspecific with _Dipodomys ordii_ and should stand as _Dipodomys ordii compactus_. Consequently a different phyletic arrangement than that proposed by Wood (_loc._ _cit._) is required. Since _D. compactus_ is more closely allied to _Prodipodomys? minor_ than _D. ordii_ is to _D. gidleyi_, it is possible that _P.? minor_ gave rise to _D. ordii_ and that _D. spectabilis_ is the end product of the phyletic trend of _D. gidleyi_ (Figure 1). The trend of phyletic development in the dipodomyines has been toward the saltatorial habit. To acquire this habit from a scampering ancestor, certain morphological modifications were necessary. Among these modifications were a lengthening of the tail, a lengthening of the hind legs, the development of a calcaneal-navicular-ectocuneiform contact instead of a calcaneal-navicular contact for additional strength in leaping, a shortening of the forelimb, an increase in size and inflation of the mastoid and tympanic portions of the skull with a consequent reduction in size of the interparietal region and the fusion of certain of the cervical vertebrae. Late Miocene (_Cupidinimus_) and Pliocene (Avawatz specimen and _Prodipodomys_) forms had acquired certain of these morphological modifications that are present in the modern genus _Dipodomys_. PHYLOGENY OF THE SPECIES OF THE GENUS Representatives of nine species of _Dipodomys_ were dissected in an attempt to determine the degree of specialization and the relative systematic position of each species. The myology was found to agree in detail as to origin, insertion and innervation with that of _Dipodomys spectabilis_ as reported by Howell (1932). The only variation noted in the muscular system was the size of the individual muscles in those animals of widely divergent body size.