SKULL.--The post-embryonic development of the skull is rapid. Animals in age groups 1 and 2 have skulls which average more than 80 per cent of the size that is here considered adult (an average size obtained from age groups 3, 4, and 5). The actual increase in size of certain cranial elements for various age groups is given in table 2.
In age group 1 the rostrum is relatively short as it is in _Neotoma micropus_ (J. A. Allen, 1894:235) and juveniles of _Peromyscus truei_ (Hoffmeister, 1951:7). The rostrum lengthens rapidly and there is a general increase in actual and relative size of the entire preorbital region; the increase after age group 3 is slower and of lesser magnitude. Changes with age in the size of the braincase are slight. In age group 1 the average depth of the braincase is 99.6 per cent of the adult size; the average breadth of the braincase is 98 per cent of the adult size, and the average width across the mastoid region is 96.4 per cent of the adult size. These dimensions indicate that the braincase reaches full size early. The zygomatic arch, however, undergoes change with age; there is a gradual increase in breadth owing to lateral bowing and a gradual lengthening which is in keeping with a general elongation of the skull anterior to the braincase.
The incisive foramina in age group 1 are short (4.0 mm), broad (2.2 mm in the middle), and taper to a point at each end. In age group 2 the foramina have elongated (4.2 mm) and are less pointed posteriorly, but there is no change in breadth. In age groups 3, 4, 5, and 6 the foramina become progressively longer (4.5 mm in age group 6), have a relatively constant breadth (2.2 mm), and become more nearly truncate anteriorly.
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 ---------------+------------+------------+------------ Number | | | examined | 4 | 13 | 33 | | | Occipitonasal | 20.5 | 21.2 | 22.0 length | 20.0 21.2 | 20.8 21.8 | 21.5 23.2 | | | Mastoid | 9.8 | 10.04 | 10.12 breadth | 9.7 10.0 | 9.6 10.4 | 9.5 10.5 | | | Length of | 8.07 | 9.02 | 9.07 zygomatic arch | 8.0 8.2 | 8.5 9.3 | 8.5 9.4 | | | Breadth of | 3.36 | 3.33 | 3.37 palate at P4 | 3.3 3.5 | 3.1 3.4 | 3.1 3.8 | | | Breadth of | 2.4 | 2.55 | 2.66 palate at M3 | 2.3 2.6 | 2.3 2.7 | 2.3 3.2 | | | Palatal | 8.67 | 8.98 | 9.38 length | 8.4 9.1 | 8.8 9.2 | 9.3 9.8 | | | Distance from | | | incisors to | 8.53 | 8.98 | 9.08 postpalatal | 8.4 8.7 | 8.5 9.5 | 9.0 9.8 notch | | | | | | Interorbital | 4.25 | 4.19 | 4.2 breadth | 4.2 4.3 | 4.0 4.4 | 4.0 4.4 | | | Average length | | | of upper | 3.2 | 3.2 | 3.21 molar series | 3.2 3.4 | 3.2 3.4 | 2.9 3.5 | | | Breadth of | 9.5 | 9.58 | 9.61 braincase | 9.3 9.7 | 9.2 9.7 | 9.1 10.0 | | | Zygomatic | 10.33 | 10.49 | 10.55 breadth | 10.0 10.7 | 10.4 10.9 | 10.1 11.2 | | | Condylobasal | 16.9 | 18.33 | 18.80 length | 16.6 17.1 | 17.4 19.2 | 18.2 19.5 ---------------+------------+------------+------------