The present study is mainly based on material collected during the 19391940 Dutch New Guinea Expedition of the "Koninklijk Nederlandsch Aardrijkskundig Genootschap" (the Royal Netherlands Geographic Society) to the region of the Wissel Lakes in central West New Guinea. Professor Dr. H. Boschma, who was the zoologist of the expedition, then obtained 753 specimens of mammals. These specimens were mainly brought together by the natives of the region, who belong to the Kepaukoe tribe. In the period from 25 September to 20 November 1939 213 marsupials, 70 bats, 465 rodents, 2 New Guinea dogs and 3 skulls of the domestic New Guinea pig were collected in the neighbourhood of Enarotali and the Araboe bivouac. Enarotali, on Paniai Lake at 3º 55' 40" S I36° 22' 6" E, was the headquarters of the zoologist of the expedition; this village is situated on the eastbank of the lake, at an altitude of 1765 m. Araboe bivouac is a bivouac on the Araboe River which empties in the N.E. angle of Paniai Lake, at an altitude of 1750 m. Because most of the material was brought in by natives, the biotopes as a rule are not known (cf. Boschma, 1943). The genus Pseudocheirus is represented in this material by two species, viz. P. corinnae Thomas and P. mayeri Rothschild & Dollman. A very fine series of 105 specimens of the latter species was obtained, which proves that it must be very abundant in the Wissel Lakes region. Also the material of P. mayeri collected by the 1938-1939 Archbold Expedition to the central part of West New Guinea could be studied. This material consists of 41 skins with skulls obtained in the central mountain range between Mt. Wilhelmina and the Idenburg River (see Archbold, Rand & Brass, 1942, map 1). Through the kindness of the late Dr. G. H. H. Tate