Museum specimens of 236 red-backed voles from the southern part of the USSR, Mongolia, and the Provinces of Inner Mongolia, Hopei, Shansi, Shensi, Kansu, Chinghai, Hupeh, and Szechwan in China were compared with 69 specimens of
Clethrionomys rufocanus from Finland. The distance from the most posterior edge of the upper 3rd molar (M3) to the most anterior point on the incisor (I-M3) correlated positively with ages determined by the developmental stages of molar roots in the year-round samples from Finland. In the samples from Finland and from eight localities in the USSR and Mongolia 46°N or more (Group A), most of the skulls measuring I-M3=14.6 mm or more did not have alveolar capsules to indicate root development. On the other hand, in the samples from 11 localities in northern China situated from 37° to 41°N, alveolar capsules indicating rootless molars were observed even in skulls where I-M3=14.6 mm or more. In the samples from 18 localities in central China from 31° to 39°N, all skulls had an I-M3 of 14.6 mm or less, and alveolar capsules were found even in adult females having prominent mammae on the skin. Adult animals larger than the smallest I-M3 class (0.3 mm interval) of the adult females in the respective localities are divided into two groups (Groups B and C) by the relation between I-M3 and interorbital width (IOW). Group B animals had larger I-M3 and smaller IOW, including the holotype of
Craseomys shanseius, whereas those of Group C had smaller I-M3 and larger IOW. Group C specimens could be further divided into two groups (C-l and C-2) based on the relation between I-M3 and tail length (TL). Given an increase in I-M3, specimens in Group C-l had a shorter TL, including the holotypes of
Microtus (Eothenomys) inez and
M. (E.) nux, whereas those of Group C-2 had a longer TL, including the holotypes of
M. (Caryomys) eva, M. (Caryomys) alcinous, and
Craseomys aquilus. Group A specimens are identified as
Clethrionomys rufocanus by the presence of roots and the age variation of molar patterns on M3, whereas, based on the absence of roots, Group B specimens are classified as
E. shanseius, Group C-l as
E. inez, and Group C-2 as
E. eva. The holotype of
M. inez jeholicus with unworn M3 is a young specimen of
E. shanseius. The distribution areas of these four species are determined and the demarcation line between C.
rufocanus and
Eothenomys is established by the Gobi Desert in Mongolia and Inner Mongolia.
E. shanseius and
E. inez are distributed sympatrically at Kolanchow, Shansi Province. Each habitat was reviewed.
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