Table 2.—Average and Extreme Measurements (in Millimeters) of Skulls of Six Age-groups in Specimens of Zapus hudsonius from Michigan.
| Age groups | 1 | 2 | 3 | 4 | 5 | 6 |
| Number examined | 4 | 13 | 33 | 14 | 3 | 3 |
| Occipitonasal length | 20.5 20.0 21.2 | 21.2 20.8 21.8 | 22.0 21.5 23.2 | 22.7 21.8 23.4 | 22.9 22.7 23.3 | 23.0 22.4 23.7 |
| Mastoid breadth | 9.8 9.7 10.0 | 10.04 9.6 10.4 | 10.12 9.5 10.5 | 10.12 9.6 10.7 | 10.3 10.0 10.8 | 10.36 10.1 10.8 |
| Length of zygomatic arch | 8.07 8.0 8.2 | 9.02 8.5 9.3 | 9.07 8.5 9.4 | 9.25 9.2 9.4 | 9.5 9.5 9.5 | 9.35 9.1 9.6 |
| Breadth of palate at P4 | 3.36 3.3 3.5 | 3.33 3.1 3.4 | 3.37 3.1 3.8 | 3.44 3.1 3.7 | 3.66 3.6 3.7 | 3.45 3.4 3.5 |
| Breadth of palate at M3 | 2.4 2.3 2.6 | 2.55 2.3 2.7 | 2.66 2.3 3.2 | 2.74 2.5 3.0 | 3.11 3.0 3.2 | 2.77 2.6 2.9 |
| Palatal length | 8.67 8.4 9.1 | 8.98 8.8 9.2 | 9.38 9.3 9.8 | 9.59 9.0 10.0 | 9.73 9.5 9.9 | 9.8 9.6 10.1 |
| Distance from incisors to postpalatal notch | 8.53 8.4 8.7 | 8.98 8.5 8.5 | 9.08 9.0 9.8 | 9.68 9.2 10.0 | 9.73 9.5 9.9 | 9.80 9.6 10.1 |
| Interorbital breadth | 4.25 4.2 4.3 | 4.19 4.0 4.4 | 4.2 4.0 4.4 | 4.2 4.0 4.4 | 4.23 4.1 4.4 | 4.2 4.2 4.2 |
| Average length of upper molar series | 3.2 3.2 3.4 | 3.2 3.2 3.4 | 3.21 2.9 3.5 | 3.22 2.9 3.5 | 3.2 3.2 3.2 | 3.16 3.1 3.2 |
| Breadth of braincase | 9.5 9.3 9.7 | 9.58 9.2 9.7 | 9.61 9.1 10.0 | 9.68 9.3 10.0 | 9.83 9.5 10.2 | 9.63 9.3 9.9 |
| Zygomatic breadth | 10.33 10.0 10.7 | 10.49 10.4 10.9 | 10.55 10.1 11.2 | 10.80 10.7 11.2 | 11.0 10.5 11.5 | 11.25 11.2 11.3 |
| Condylobasal length | 16.9 16.6 17.1 | 18.33 17.4 19.2 | 18.80 18.2 19.5 | 19.33 18.5 19.9 | 19.6 19.4 19.8 | 19.9 19.5 20.3 |
Individual Variation
Measurements of external parts in Zapus are more variable than are measurements of most parts of the skull. As Hoffmeister (1951:16) points out for Peromyscus truei, this variation in external features results in part from “the difficulties in accurately measuring soft parts of the anatomy” and also from inconsistencies on the part of collectors in making these measurements.
A comparison of coefficients of variation (see [table 3]) for cranial measurements between populations of like age and sex for the species Z. hudsonius, Z. princeps, and Z. trinotatus shows that variation of approximately the same degree is recorded in corresponding elements in all species; that is to say, structures which are most variable individually in Z. princeps are also most variable in Z. trinotatus and Z. hudsonius.
Individual variation in the occlusal pattern of the molariform teeth is slight. In several specimens, however, the re-entrant fold is absent from the lingual surface of M1. Teeth in addition to the normal number were recorded for five specimens. In all instances they are in the upper dentition and usually at the posterior end of the maxillary tooth-row. In each of four specimens (KU No. 34852, KU No. 32852, MVZ No. 52105, all Z. princeps, and USBS No. 22921, Z. hudsonius), there is only a single additional tooth. One individual (USBS No. 264388, Z. princeps) possessed two extra molars, one in each maxillary tooth-row. The extra teeth vary in size from those which are only slightly smaller than the adjacent normal molars to those which are simple, peglike structures. In four of the five animals the extra teeth are posterior to the normal M3; in the fifth (MVZ No. 52105) the added tooth is anteriormedial to M3.
Table 3.—Coefficients of Variation for Dimensions of Corresponding Parts of the Skull of Three Species of Zapus. The Specimens of Zapus hudsonius are from Menominee and Keweenaw counties, Michigan, the Zapus princeps are from the Vicinity of Encampment, Wyoming, and the Zapus trinotatus from Huntingdon, British Columbia.