[Illustration: FIG. 1. Ventral views of skulls and left maxillary tooth-rows of two species of the genus _Apodemus_. _a._ _Apodemus flavicollis flavicollis_ (Melchior), Lolland, Denmark, adult [Male], No. 141691 USNM, ×2. _b._ _Apodemus flavicollis flavicollis_ (Melchior), Mauseklippe, Germany, young [Male], No. 112895 USNM, ×10. _c._ _Apodemus peninsulae peninsulae_ (Thomas), Central Nat'l Forest, near Pup'yong-ni, 200 m., Korea, subadult [Female], No. 300650 USNM, ×10. _d._ _Apodemus peninsulae peninsulae_ (Thomas), 6 mi. S Yongdongp'o, Korea, adult [Male], No. 299554 USNM, ×2. In comparing the ventral views of skulls note especially the size and location of incisive foramina and posterior palatine foramina as well as the breadth of mesopterygoid fossae. In comparing the left maxillary tooth-rows note especially the size of M3 and the reduced posterointernal cusp on Ml in _A. peninsulae_.] More recently, Ellerman (1949:32) and Ellerman and Morrison-Scott (1951:566) have arranged _peninsulae_ as a subspecies of _Apodemus flavicollis_ under the assumption that all the members of the subgenus _Sylvaemus_ on the eastern Asiatic mainland are subspecies of one or another of the species of western Europe, _A. flavicollis_ or _A. sylvaticus_. Ellerman (in Ellerman and Morrison-Scott, 1951:564) states: "The majority of the forms I distribute in a somewhat arbitrary manner between _sylvaticus_, average smaller skull, and _flavicollis_, average larger skull; occurring together nearly throughout the Palaearctic. I feel fairly sure that there are some errors of judgment in my arrangement, and equally sure that there is no other way to define species in this very large and difficult group." I have compared the specimens of _peninsulae_ available to me from central and southern Korea with specimens of _A. f. flavicollis_ from Denmark, Germany and Sweden and find, although the two are similar in many ways, that _peninsulae_ differs from _flavicollis_ in several important characters: Mammae 1-2=6 in _flavicollis_, and 2-2=8 in _peninsulae_; incisive foramina reaching level of alveoli of M1, or nearly so, in _flavicollis_, but ending conspicuously short of that level in _peninsulae_; posterior palatine foramina large in _flavicollis_ and opposite a point where M1 and M2 meet, but small in _peninsulae_ and situated farther back on the palate, opposite M2. Moreover, _peninsulae_ lacks the characteristic buffy throat patch of _flavicollis_, has a much reduced posterointernal cusp on the M1, a relatively (frequently actually) larger M3 and, on the average, a broader mesopterygoid fossa. In view of these differences, all of which appear to be constant, I consider _peninsulae_ specifically distinct from _flavicollis_. Throughout its known geographic range (see below) _peninsulae_ is evidently confined to wooded terrain, either scrub or brush types or forested areas, and the vernacular name wood mouse, therefore, seems appropriate for this species.