MERRIAMI GROUP.--Composed of the subspecies of _Dipodomys merriami_, _Dipodomys nitratoides_ and _Dipodomys phillipsii_, and the species _Dipodomys platycephalus_, _Dipodomys margaritae_, _Dipodomys insularis_, _Dipodomys mitchelli_, _Dipodomys ornatus_ and _Dipodomys elator_. I have not examined five of these species. However, the indices and characters of the viscera indicate that the three species first mentioned are closely allied. Owing to the lack of known intergradation between the three, I judge that they should be retained as full species, but the difference in degree of morphological specialization is no more than would be expected between subspecies. I have examined no specimens of _Dipodomys elator_ but from what I know of its morphology, I think that Grinnell better indicated its relations in allying it with _D. phillipsii_ than did Davis in erecting a new group for it on the basis of linear measurements. DESERTI GROUP.--Composed of _Dipodomys deserti_ which has only two subspecies. In all morphological respects, _D. deserti_ is the most specialized species in the genus as shown by the reduced number (4) of toes on the hind foot, the bilateral arrangement of the viscera, the extreme development of the auditory region of the skull and by developing, early in life, the hiatus in the enamel wall of each molariform tooth. The parallel arrangement below emphasizes the differences and similarities between Grinnell's (1922) arrangement and the one proposed in the present paper. Grinnell's arrangement Present arrangement HEERMANNI GROUP HEERMANNI GROUP _Dipodomys heermanni_ _Dipodomys heermanni_ _Dipodomys morroensis_ _Dipodomys agilis_ _Dipodomys mohavensis_ _Dipodomys ingens_ _Dipodomys leucogenys_ _Dipodomys venustus_ _Dipodomys panamintinus_ _Dipodomys elephantinus_ _Dipodomys stephensi_ _Dipodomys ingens_ SPECTABILIS GROUP SPECTABILIS GROUP _Dipodomys spectabilis_ _Dipodomys spectabilis_ _Dipodomys nelsoni_ _Dipodomys nelsoni_ PHILLIPSII GROUP NOW IN MERRIAMI GROUP BELOW _Dipodomys phillipsii_ _Dipodomys perotensis_ _Dipodomys ornatus_ _Dipodomys elator_