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). -----------------------+-----------------------+----------------------- _Eozapus_ | _Zapus_ | _Napaeozapus_ -----------------------+-----------------------+----------------------- P4--Small | Smaller | Absent | | M1--Four wide labial | Four moderately | Three narrow labial re-entrant folds | narrow labial | re-entrant folds of equal length; | re-entrant folds | of unequal length, paracone and | of unequal length; | 1st long, 2d and 3d metacone largest | 1st and 3d longer | shorter; paracone cusps; anterior | than 2d, 4th | and metacone larger cingulum large. | shortest; paracone | than in _Zapus_ and | smaller than in | _Eozapus_; anterior | _Eozapus_; metacone | cingulum absent. | largest cusp; | | anterior cingulum | | small. | | | | | M2--Four wide labial | Four moderately | Narrow labial re-entrant folds; | narrow labial | re-entrant folds, 2d short, others | re-entrant folds | variable in number, of equal length | of unequal length, | often as many as 6;