Heteromyids are known from the Chadron formation, of early Oligocene age, in which a single tooth was found. In the Orellan stage of the mid-Oligocene where the genus _Heliscomys_ occurs, it is notably generalized, in comparison with other members of the family, but it may not be ancestral at all. The lower premolar is tricuspidate and the first and second molars are quadritubercular with a broad cingulum. The teeth are bunodont and brachydont, with the cusps not uniting to form lophs. Wood (1935:78) shows _Mookomys formicorum_ (from the Arikeean) as the next heteromyid in the evolutionary sequence and postulates that this species arose from _Heliscomys gregoryi_. _Mookomys_ is judged by Wood to be the common ancestral form of the perognathines and the dipodomyines. _Cupidinimus_, the genus next in line, is characterized by smooth upper incisors; lower molars with incipient H-pattern; cheek teeth progressively hypsodont and lophate (but always rooted); and calcaneal-navicular articulation. The time range of this genus is from the late Miocene (Niobrara River, Local Fauna) of Nebraska to the medial Pliocene, Thousand Creek (Hemphillian) of Nevada. Hibbard (1937:462) described _Dipodomys kansensis_ from the Ogallala formation (Hemphillian age) of Kansas. He redescribed his species, and made it the type of the new genus _Prodipodomys_ (Hibbard, 1939:458), differentiating it from _Dipodomys_ on the basis of the three-rooted p4, double-rooted m1 and m2 and the single rooted m3. It is shown to be closely allied to _Dipodomys_ by the form and position of a large foramen posterior and labial to m3, and by the development of the masseteric ridge. The next youngest heteromyid fossils which have been described are of the genus _Prodipodomys_? from Arizona. Gidley (1922:123) described _Dipodomys minor_ from the Benson (Blancan) which Gazin (1942:486) refers to the genus _Prodipodomys_?. Wood (1935:156) described _Dipodomys gidleyi_ from the Curtis (Pleistocene). Both of these species are primitive as regards dentition; that is to say, the enamel ring of the tooth is complete and lacks any sign of a break. The limb bones of _D. gidleyi_ show lesser saltatorial ability, and therefore appear to be more primitive, than those of any living _Dipodomys_.