Pseudamnicola confusa (Frauenfeld, 1863), a common Mollusc in the Biesbosch Pseudamnicola confusa was first collected in the Netherlands on the island of Rozenburg in 1944 as an empty shell and one living specimen from the same island in 1945. For fourteen years the mollusc was not seen again and it was doubted whether it should be considered as belonging to the Dutch fauna as VAN REGTEREN ALTENA had published in 1958. In a zoological inventory of the Biesbosch (August 1958), necessituated by the irreversible changes which will be brought about by the enclosure of the sea arms in the province of Zeeland and which would affect nature reserves under supervision of the Department for Nature Conservation of the State Forestry, both living and dead specimens were found and Pseudamnicola confusa was shown to appear in connection with a member of the plant associations of the Bidention tripartiti Nordhagen, 1940 (cfr. ZONNEVELD, 1960): Polygonum hydropiper, Nasturtium palustre and Bidens spec., on the mud surface between the plants. It is suggested that the inaccessibility of the muddy biotope between high- and low waterline in the delta was the main reason for the overlooking of this mollusc and, secondly, the unexpectedness of a new species appearing in our country. The occurrence of the species was first investigated by the examination of the thanatotopes in the Biesbosch, -i.e. by the examination of large numbers of dead shells brought together by natural causes, e.g. currents or wind. From this it appears that if the shell of a given species is found in a thanatotope it is a good indication that the living mollusc will also be found on closer examination of a biotope in the neighbourhood of the thanatotope. A further point is that if only dead specimens of a given species are found in a biotope, then the biotope has not been studied closely enough, for the living mollusc should also be found there on further investigation unless a good reason can be given for its extinction or for its appearance as a dead shell only.