Fig. 14. Dorsal views of skulls of voles of known age. (Ages 11⁄2, 21⁄2, 3, 31⁄2, 4, 41⁄2, 6 and 12 months). All × 3.

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Fig. 15. Palatal views of skulls of voles of known age. (Ages 11⁄2, 21⁄2, 3, 31⁄2, 4, 41⁄2, 6 and 12 months). All × 3.

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From a palatal view ([Fig. 15]) the skulls of voles also showed age variation which was apparent but not easily correlated with precise age. The median ridge on the basioccipital bone increases in prominence in older voles. The shape of the posterior margin of the palatine bones changes from a V-shape to a U-shape. On the skull of the oldest (12 months) vole the pterygoid processes are firmly fused to the bullae, a condition not found in any of the other specimens. The anterior spine of the palatine approaches the posterior projection of the premaxillae more closely as age increases and, in the oldest vole is firmly attached and forms a complete partition separating the incisive foramina.

Tooth wear during the life of a vole causes a considerable variation in the enamel patterns, especially of the third upper molar. Howell (1924:1012) considered such variation to be independent of age, but Hinton (1926:103) related the changes to age and interpreted them as a recapitulation of the evolution of microtine molars. In my series, an indentation on the medial margin of the posterior loop of the third upper molar seemed to be related to age. This indentation was absent in the youngest vole (one and one-half months), absent or indefinite in those voles less than 31⁄2 months of age, and progressively more marked in the older voles.