_Eozapus_ is more closely related to _Pliozapus_ than to either _Zapus_ or _Napaeozapus_ (Wilson, 1936:32) but all four genera are in the subfamily Zapodinae. Stehlin and Schaub (op. cit.:158 and 311) relate _Eozapus_ to the subfamily Sicistinae on the basis of similarity in the occlusal pattern of the cheek-teeth of _Eozapus_ and various sicistines. Stehlin and Schaub do not consider other structures such as the elongate hind limbs, the shape of malleus and incus, and the shape of the baculum, in which there is close resemblance to the Zapodinae. It is these structural similarities as well as those, pointed out by Wilson (_loc. cit._), in dentition that leads me to place _Eozapus_ in the subfamily Zapodinae. The early Pleistocene _Zapus rinkeri_ Hibbard shows that the _Zapus_ stage of development had already been achieved perhaps as early as the late Pliocene. Hibbard (1951:352) thought that _Zapus rinkeri_ was not structurally intermediate between _Pliozapus_ and any Recent species of _Zapus_; although the teeth of _Z. rinkeri_ have the broader, shallower, re-entrant folds of _Pliozapus_, these teeth are higher crowned and have an occlusal pattern resembling that of the Recent species of _Zapus_. The middle Pleistocene species, _Zapus burti_ Hibbard, progressed essentially to the structural level of the Recent _Zapus hudsonius_, but the molars were more brachydont, broader crowned, and their enamel folds less crowded. Pleistocene material of pre-Wisconsin age obtained from cave deposits in Pennsylvania and Maryland is most nearly like _Zapus hudsonius_. One such cave deposit in Maryland contained an example of the Recent genus _Napaeozapus_, indicating that its history dates from at least middle Pleistocene time. The Asiatic Recent Genus, _Eozapus_, has not progressed much beyond the Pliocene stage in zapidine evolution if _Pliozapus_ be taken as a standard; the North American Recent Genus _Zapus_ essentially achieved its present form by early Pleistocene times, and the Recent Genus _Napaeozapus_ achieved its more progressive structure by middle Pleistocene times. Perhaps _Pliozapus_ and _Eozapus_ represent one phyletic line and _Zapus_ and _Napaeozapus_ a second line, both of which lines evolved from a pre-zapidine stock in the Miocene. As mentioned earlier, Wilson (1936) thinks that _Pliozapus_ is not directly ancestral to _Eozapus_. Possibly these two genera diverged at an early date; nevertheless, they are closely related primitive forms. _Zapus_ and _Napaeozapus_ closely resemble each other and both are structurally advanced; _Napaeozapus_ seems to have differentiated at a more rapid rate.