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. NONGEOGRAPHIC VARIATION 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.