Several heteromyids which have not been assigned to any genus are known. Wilson (1939:36-37) recognized some from the Avawatz (Clarendonian) and the Ricardo (Clarendonian). Another, possibly of the genus _Diprionomys_?, from the Barstow (Barstovian) was described by Wood (1935:197) as follows: "The general shape of the tooth as figured strongly suggests either one of the most advanced species of _Dipodomys_ or else a Geomyid.... It is much more advanced than are any known contemporary heteromyids, and compares fairly well with such late Tertiary and Pleistocene geomyids as have been described. It certainly is not referable to any known heteromyid genus other than _Dipodomys_, and should probably be called a Geomyid." Wilson (_loc. cit._) refers to these specimens as Dipodomyine (?) n. gen. and sp. If these specimens referred to by Wood and Wilson are true heteromyids then a change in the phylogenetic scheme proposed by Wood (1935) would be necessary. Wilson (_loc. cit._) says, referring to the Avawatz specimen, "The cheek teeth are very hypsodont but are apparently not persistent in growth,... Wide enamel breaks are present in M/1 dividing the enamel into anterior and posterior bands. The enamel of P/4 is complete in the present stage of wear, but an examination of the tooth indicates that breaks would develop with additional attrition at the buccal and lingual margins of the metalophid, and at the buccal border of the protolophid. The incisor is of the slender heteromyid type." Wood (1935:118) in referring to the ancestry of _Cupidinimus_ with regard to the grooving of the incisors says: "The philosophy of evolution which would prohibit its derivation from _Mookomys_, because of the grooved incisors in the latter genus, would require a separate line leading back at least to the Lower Miocene." In view of the above statements, it is conceivable that additional material will be found carrying the dipodomyine line back into the early Miocene. Perhaps the line involving _Mookomys_ and _Cupidinimus_ which was regarded by Wood as the line of descent, is merely an aberrant side branch that parallels in its structures the main line of evolution of the dipodomyines (Figure 1). [Illustration: FIG. 1. Phylogeny of the Dipodomyines (modified after Wood, 1935).] As Wilson (1939:37) says: "Indeed it is hard to recognize such a form as _Cupidinimus nebraskensis_ as directly ancestral to _Dipodomys_ in view of the occurrence of the much more advanced Avawatz specimen in deposits that are at most only slightly later than those in which the former is found. The kangaroo rats were apparently much farther along in their development by lower Pliocene time than heretofore supposed."