[Illustration: FIG. 13. Graphs of the condylobasilar lengths, body lengths and weights of a series of voles of known age. Within each age group, the youngest vole is on the left in the graphs.] When other cranial measurements, and ratios of pairs of measurements, were plotted in the same order, individual variation obscured some of the variation due to age and the curves resembled those of weight and length of body rather than that of condylobasilar length. When the cranial measurements were averaged for the age groups the curves showed a relationship to age but the relationship of mean measurements is of little use in determining the age of individual specimens. The data described above indicated that a study of the relationship of the condylobasilar length and age in a large sample might provide useful information. Anyone who has examined mammalian skulls knows of many other characters which vary with age but which are difficult to measure and describe with precision. Figs 14 and 15 are drawings of skulls of voles of known age. The most obvious change, related to aging, evident in the dorsal view of the skulls (Fig. 14) is the increasing prominence and closer approximation of the temporal ridges in older specimens. The lambdoidal ridge is also more prominent in older voles, and their skulls have a generally rougher and more angular appearance. The individual variation evident in these ridges is probably due to variations in the development of the muscles operating the jaws (Howell, 1924:1003). There is an increased flattening of the roof of the skull of older voles. [Illustration: 1-1/2 months 2-1/2 months 3 months 3-1/2 months 4 months 4-1/2 months 6 months 12 months All x 3. FIG. 14. Dorsal views of skulls of voles of known age.] [Illustration: 1-1/2 months 2-1/2 months 3 months 3-1/2 months 4 months 4-1/2 months 6 months 12 months All x 3. FIG. 15. Palatal views of skulls of voles of known age.] 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.