Examination was made of 13,000 individuals of Clethrionomys plareolus (S c h r e b e r, 1780) and 4,000 individuals of Clethrionomys rutillus (Pallas, 1779) obtained in the taiga forests covering the region between the rivers Wiatka and Kama, over a period of 5 years, most often from May to August. Several groups were distinguished on the basis of differences in length of root and height of crown of M 2. After comparing the times of beginning and end of reproduction, periods of appearance of the maximum number of litters in a population with a defined type of tooth structure, it proved possible to establish the absolute age of each of the groups distinguished. A key is given to identification of the absolute age of the two species of vole examined. Up to the second month of life, the teeth consist of prisma only, the neck of the tooth not being formed until the voles are two months old. At the age of 3-4 months the roots form less than 1 /* of the whole height of the tooth. At the age 5-6 months this relation is Vi, 7-8 months-9-10 months-Vs and at the age of 11-12 months-2 h. In the case of animals over one years old only the roots of the tooth remain, or part of the neck may still be seen.
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