Two new species, Mojavemys galushai and Phelosaccomys neomexicanus, are described from the Barstovian of New Mexico. Both of these species are assigned to a new subfamily, the Mojavemyinae. It is uncertain as to whether this new subfamily should be referred to the Heteromyidae or Geomyidae. Both Mojavemys an Phelosaccomys retain some heteromyid characteristics while developing advanced traits of geomyids. It is possible that these genera represent transitional forms between a heteromyid ancestor and advanced geomyines, thus deriving the Geomyidae directly from the Heteromyidae sometime in the middle Tertiary. It is also possible that the Mojavemyinae are a subfamily of heteromyids only convergent with geomyids. A third possibility is that the mojavemyines are among a group of middle Tertiary geomyoids from which both Recent heteromyids and geomyids were derived.