Type genus of Soricidae. The systematic relationships of a large number of Holarctic species were studied by George (1988); her proposals for subgeneric allocation are mainly followed here. Keys and/or reviews are available for the species of various geographical areas: Canada (van Zyll de Jong, 1983a); North and Middle America (Carraway, 1990, 1995; Junge and Hoffmann, 1981); China (Hoffmann, 1987); Siberia (Yudin, 1989); and Europe (Niethammer and Krapp, 1990). Microsorex was formerly regarded as a full genus, then reduced to a subgenus of Sorex by Diersing (1980b), and is now regarded as a synonym of subgenus Otisorex (see George, 1988). Besides subgenera a number of species groups have been distinguished such as the araneus arcticus group (Hausser et al., 1985; Meylan and Hausser, 1973), the cinereus group (van Zyll de Jong, 1991b), and the vagrans group (Carraway, 1990), the boundaries and contents of which are still highly controversial (see Demboski and Cook, 2003). Old World species of Sorex were reviewed by Dannelid (1991b) who provided a phylogenetic hypothesis of relationships. Fumagalli et al. (1996, 1999), Ohdachi et al. (1997a, 2001), and Demboski and Cook (2001, 2003), among others, provided important genetic studies of the phylogeny and geographical variation of certain species complexes. Zima et al. (1998) reviewed data on chromosomal variation in Sorex and suggested species groups that are not always congruent with clusters based on genetical or morphological data.
Not all species can be allocated to subgenera or species groups. There exist various proposals for species groups based on chromosomes, genetics, or morphology, and not all are congruent. Many species have never been studied properly, and the genus offers a wide and promising field for further research. Not all taxon names can be allocated to species. The enigmatic Sorex fimbripes Bachman, 1837 cannot be identified in the absence of the holotype specimen or new material from the type locality (Handley and Varn, 1994). Also the identities of the taxa Sorex pusillus Gmelin, 1774; Sorex dorichurus Kishida, 1937; Sorex longiusculus Kishida, 1928 (? nomen nudum); Sorex longicaudatus Kishida, 1930 (nomen nudum, not Okhotina, 1993); and Sorex longicaudatus Yoshikura, 1956 (not Okhotina, 1993) remain unsolved.