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.

The absence of P4 in Napaeozapus would lead one to suspect that this genus has evolved at a more rapid rate than the historically older Zapus and Eozapus which still retain this structure. The small size of P4, even in the primitive Eozapus, indicates that it has long been of little use to the mouse. An even greater reduction of P4 in the more complex dentition of Zapus argues for complete loss of this tooth as the next step in specialization, such as is seen in the more progressive Napaeozapus. The following parallel columns show selected differences between the occlusal patterns of the cheek-teeth of the three genera:

Baculum.—The baculum (os penis) of Eozapus is known to me only from Vinogradov’s (1925) figures of the dorsal and lateral aspects. The proximal end (base) is laterally expanded, and the shaft tapers gradually toward the distal end where it expands abruptly into the spade-shaped tip. In lateral aspect the bone is relatively thick; it is curved downward slightly from the proximal end to the base of the tip where it curves upward to a rounded point.

The baculum of Zapus differs from that of Eozapus as follows: base less expanded horizontally; shaft slenderer; distal end less spade-shaped except in Z. trinotatus. The tip is less expanded in Z. princeps and is still less so in Z. hudsonius. In Napaeozapus the tip is lanceolate, the base is narrow, and in lateral view the shaft is slender and curved (see figs. 22-31).