===============+============+============+============ Age groups | 4 | 5 | 6 ---------------+------------+------------+------------ Number | | | examined | 14 | 3 | 3 | | | Occipitonasal | 22.7 | 22.9 | 23.0 length | 21.8 23.4 | 22.7 23.3 | 22.4 23.7 | | | Mastoid | 10.12 | 10.3 | 10.36 breadth | 9.6 10.7 | 10.0 10.8 | 10.1 10.8 | | | Length of | 9.25 | 9.5 | 9.35 zygomatic arch | 9.2 9.4 | 9.5 9.5 | 9.1 9.6 | | | Breadth of | 3.44 | 3.66 | 3.45 palate at P4 | 3.1 3.7 | 3.6 3.7 | 3.4 3.5 | | | Breadth of | 2.74 | 3.11 | 2.77 palate at M3 | 2.5 3.0 | 3.0 3.2 | 2.6 2.9 | | | Palatal | 9.59 | 9.73 | 9.8 length | 9.0 10.0 | 9.5 9.9 | 9.6 10.1 | | | Distance from | | | incisors to | 9.68 | 9.73 | 9.80 postpalatal | 9.2 10.0 | 9.5 9.9 | 9.6 10.1 notch | | | | | | Interorbital | 4.2 | 4.23 | 4.2 breadth | 4.0 4.4 | 4.1 4.4 | 4.2 4.2 | | | Average length | | | of upper | 3.22 | 3.2 | 3.16 molar series | 2.9 3.5 | 3.2 3.2 | 3.1 3.2 | | | Breadth of | 9.68 | 9.83 | 9.63 braincase | 9.3 10.0 | 9.5 10.2 | 9.3 9.9 | | | Zygomatic | 10.80 | 11.0 | 11.25 breadth | 10.7 11.2 | 10.5 11.5 | 11.2 11.3 | | | Condylobasal | 19.33 | 19.6 | 19.9 length | 18.5 19.9 | 19.4 19.8 | 19.5 20.3 ---------------+------------+------------+------------
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.