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.
A knowledge of variation resulting from age, individual, or secondary sexual differences, as opposed to geographic variation between two or more populations of a single species is important in determining the reliability of taxonomic characters.
The largest population-sample of Zapus available to me for the study of nongeographic variation was 63 individuals from various localities in Keweenaw and Menominee counties, Michigan. Thirty-nine were females and 24 were males. It is on these specimens that this discussion is based.
Age Variation
Teeth.—The teeth provide a valuable standard for age determination in that they wear at a measurable rate. The molars erupt in sequence from front to back, and wear shows first on M1 and last on M3. The peglike permanent P4, of which I have not seen the deciduous precursor, receives wear at the same time that the molars are being worn. Wear proceeds at approximately the same rate in the teeth of both the upper jaws and lower jaws.