[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.
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.
SKULL.--Some of the structures useful for separating taxonomic entities may have little or no biological significance to the animals in nature. Characters mentioned by me are chosen simply for their significance taxonomically. The zygomata vary in degree of lateral bowing, being widely bowed in _Z. princeps_ and _Z. trinotatus_, and less so in _Z. hudsonius_. Differences in zygomatic breadth owing to the degree of bowing are an aid in differentiating subspecies. The length of the skull from the occipital condyles to the tip of the longest nasal bone is useful in separating _Z. hudsonius_ from _Z. trinotatus_ and _Z. princeps_. The narrowness of the base of the zygomatic process of the squamosal is useful in distinguishing between _Z. hudsonius_ and _Z. princeps_, but shows no variation of subspecific worth. The shape and dimensions of the incisive foramina provide specific and subspecific characters. The position of the anterior margin of the postpalatal notch, in relation to the last molars, provides subspecific characters in _Z. princeps_. In the species _Z. princeps_ the median projection on the inferior ramus of the zygomatic process of the maxillary is absent in some subspecies, small in others, and large in some. Shape and inflation of the auditory bullae, shape of the pterygoid fossae, and shape of the nasals are useful in determining specific and subspecific relationships.
TEETH.--The alveolar length of the upper maxillary tooth-rows aids in distinguishing _Z. hudsonius_ from _Z. princeps_ and _Z. trinotatus_. Nearly parallel versus anteriorly divergent upper tooth-rows is a subspecific difference in _Z. princeps_. Variations in the dimensions of P4 and M1 aid in estimating the relationships of species. The occlusal pattern shows little variation and was of no use in separating species.
