===============+============+============+============ 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 ---------------+------------+------------+------------ 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.