FIGS. 12-15. _Napaeozapus i. insignis_, adult, male, No. 41109 KU, Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County, Massachusetts.] [Illustration: FIGS. 16-21. Occlusal views of upper and lower right cheek-teeth, of the three Recent genera of the subfamily Zapodinae. × 12-1/2. FIGS. 16 and 19. _Eozapus s. vicinus_, adult (age group 3), male, No. 240762 USNM, Lanchow, Kansu, China. FIGS. 17 and 20. _Zapus h. alascensis_, adult (age group 2), female, No. 29073 KU, E side Chilkat River, 9 mi. W and 4 mi. N Haines, Alaska. FIGS. 18 and 21. _Napaeozapus i. insignis_, adult (age group 3), male, No. 41109 KU, Shutsburg Rd., at Roaring Creek, 600 ft., Franklin County, Massachusetts. Note especially the variation in complexity of occlusal pattern, width of re-entrant folds, and degree of tubercularity.] The remarkable similarity of the body form, post-cranial skeleton, mandibular rami, and general structure of the cranium of _Zapus_, _Napaeozapus_, and _Eozapus_ indicate their relationship (see figs. 4-15); however, dissimilarity between the groups in the dentition (tooth number and occlusal pattern), bacula, and ear ossicles provides basis for considering them distinct genera. As pointed out earlier, _Zapus_ and _Napaeozapus_ appear to be more closely related and progressive and the Asiatic _Eozapus_ somewhat removed and less progressive. _Teeth._--According to the complexity in dental pattern and in number and size of the cheek-teeth, these genera can be arranged in a structurally progressive series with _Eozapus_ showing the least complexity and _Napaeozapus_ the most (see figs. 16-21). There are three distinct molar patterns; one is simple (_Eozapus_) and the others (_Zapus_ and _Napaeozapus_) are more complex. The complexity is greatest in _Napaeozapus_, which is characterized by numerous additional flexures in the enamel and dentine. The simplicity of the molars of _Eozapus_ is evident in the tuberculate rather than flat-crowned occlusal surface; the wide, simple, re-entrant bays; the small (or sometimes absent) anteroconid; and the essentially quadritubercular nature of the teeth. The molars of _Zapus_ and _Napaeozapus_ are flat crowned; however, _Zapus_ has wider and fewer re-entrant bays, a smaller anteroconid, and less complexity in the occlusal pattern. The characteristics of the molar teeth would tend to indicate a close relationship between _Zapus_ and _Napaeozapus_ and to place _Eozapus_ as primitive.