Arvicolini. Volemys circumscribes two species restricted to the alpine-subalpine zone in S China that may be part of an older fauna (Lawrence, 1982). Both have traditionally been placed in Microtus, although their phylogenetic relationships were regarded as obscure or equivocal (G. M. Allen, 1940; Zagorodnyuk, 1990). Corbet and Hill (1992) did not recognize Volemys, and Pavlinov et al. (1995a) arranged it as a subgenus of Microtus. In his generic description, Zagorodnyuk (1990) added two other species: Taiwanese kikuchii and clarkei from the mountains of Yunnan, N Burma, and SE Tibet. Phylogenetic analysis of cytochrome b sequences demonstrated that kikuchii is sister-species of M. oeconomus (Conroy and Cook, 2000a), within a clade embracing M. montebelli, M. middendorffii, and M. fortis, all members of Microtus, subgenus Alexandromys (see account of M. kikuchii). In reassessing the characteristics of clarkei (see that account), we find that it too is morphologically unlike V. millescens and V. musseri and instead resembles M. fortis, another species in the subgenus Alexandromys.
Although Zagorodnyuk (1990) formally proposed Volemys, its diagnostic traits had been previewed by Lawrence (1982), who concluded that musseri and millescens do not morphologically fit with other species-groups of Microtus. Those traits are long tail relative to head and body; smooth and flattened cranium; slim dentary with a low ramus; inflated auditory bullae; M1-2 with a large posterolingual triangle confluent with the opposite labial triangle and forming an inverted chevron-shaped lamina (in V. musseri; the configuration characterizes only the M2 in V. millescens); m1 with only four closed triangles anterior to the posterior lamina, the anterior triangles confluent and with the large anteroconid cap; anterior lamina of each m2 is chevron-shaped and formed by confluent lingual and labial triangles. Volemys millescens is smaller than V. musseri and differs in pelage coloration, but is similar in molar patterns except for the lack of an M1 posterolingual triangle. Their dental patterns are unique among species of Microtus, and in combination with the distinctive cranial and mandibular conformations and external proportions, isolate millicens and musseri from any other species-group of Microtus or any other genus of Arvicolini.
Species of Volemys, along with Neodon irene and certain Eothenomys, are the only voles recorded from the high mountains of W Sichuan, China, where no species of true Microtus has been found. Microtus fortis does occur in SE Sichuan but inhabits river valleys at low altitudes, and M. clarkei is known only from above 3400 m in W Yunnan and N Burma (see those accounts).