Ever since his first experience with the remarkable “Upper Oligocene” molluscan fauna of the Isle of Buton, the present writer has endeavoured to find more convincing evidence for its age. One of the most tempting problems was why this fauna showed so few relationships to other fossil faunas or to the living mollusca (See Martin, 1933, 1935; Beets, 1942, a, d). Since the only firm point emerging from a number of more or less confusing data was that the closest relationships existed with the Neogene fauna of the East Indies, the writer started extensive comparisons with a number of undescribed fossil collections from that region kept in Netherlands museums (Leyden Geological Museum, Delft Mining Institute, Utrecht Geological Institute). Meanwhile, additional fossils from Buton at first still believed to be of an Oligocene age were received in 1943 from both the “Rijkswegenbouw-Laboratorium”, The Hague, and the “Naturhistorisches Museum”, Basle, the latter fauna accompanied by notes concerning the locality compiled by its collector, Dr. F. Weber, Lugano. The comparisons mentioned above bore their first fruits late in 1943 and earljr in 1944 when species described from Buton were discovered in an undescribed collection of mollusca from East-Borneo which apparently indicated unusually deep water deposition. It soon became apparent that the “oligocene” mollusca from Buton too should be considered as a “deep water” fauna. This seemed to explain a number of puzzling facts which up till that time did not fit the picture of Tertiary faunal development in the East Indies. Moreover, it appeared that the age of the fauna most probably was to be considered as Mio-Pliocene. Following researches with the aid of the collections in the Zoological Museum, Amsterdam, the Leyden Museum of Natural History and the British Museum (Natural History) confirmed the above revised views.