This paper presents the results of the examination of a fairly big collection of mollusca from the island of Mandul, north of Tarakan, East-Borneo. The material was collected by Dr. Van Holst Pellekaan while investigating the geology of Mandul in the service of the “Bataafsche Petroleum Maatschappij” (Royal Dutch/Shell). It was sent to Prof. K. Martin of Leyden for closer examination, and afterwards was embodied into the collections of the Leyden Geological Museum. Prof. Martin recorded the results of his preliminary examination, which excluded the bivalves, in a report to the “Bataafsche”, dated 12th January 1917. He came to the conclusion that the fossils were of a Pliocene age.