In _Zapus princeps_ new hair appears first on the mid-dorsal surface between the scapulae. From this starting point molt progresses anteriorly, laterally, and posteriorly. Progress over the head is rapid; the head receives its new hair sooner than the caudal region. Molt moves progressively nearer to the base of the tail and progressively nearer to the mid-ventral surface. The rump is the last area to complete its molt (see figs. 40 and 41). The progress of molt in _Z. princeps_ might be likened to the flow of a drop of paint on the curved surface of a ball where the paint flows in all directions but is speeded at one point and slowed at the opposite by a slight tilting of the ball from the horizontal. In the species _Zapus trinotatus_ new hair appears simultaneously on the anterior, dorsal surface of the nose and on the mid-dorsal surface between the scapulae. In this respect the progress of molt of _Z. trinotatus_ resembles that of _Z. hudsonius_. From these starting points molt progresses rapidly over the head, the molt moving anteriorly from the shoulders and posteriorly from the nose with the result that it covers the dorsal surface of the head; hair then appears on the cheeks and sides of the neck. The progress of molt on the remaining areas of the body is comparable to that of _Z. princeps_; molt progresses toward the tail and toward the mid-ventral line. The rump, as in _Z. princeps_, is the last area to complete its molt (see figs. 38 and 39). [Illustration: FIGS. 38-43. Diagrams showing differences in progress of molt in the three species of the genus _Zapus_. All approximately 1/2 natural size. Figs. 38, 40 and 42 lateral view. Figs. 39, 41 and 43 dorsal view. FIGS. 38 and 39. _Zapus trinotatus._ FIGS. 40 and 41. _Zapus princeps._ FIGS. 42 and 43. _Zapus hudsonius._] BACULUM.--The general shape and dimensions of the baculum (os penis) provide characters of taxonomic value for the species of _Zapus_ (see figs. 23-25 and figs. 28-30). Three measurements--length, transverse diameter at the base, and transverse diameter at the tip--are easily obtained and are diagnostic. The bacula of all species are somewhat curved. The measurement of length used by me does not represent the actual length of the bone, but instead the chords of the arcs involved.