During 1948 an investigation of the mollusk-fauna of certain pools and minecraters in the dunes between Den Helder and Schoorl (Prov. of North-Holland, Netherlands) took place. The succession of the snail-population, so different in the old dune pools compared with the minecraters of more recent origin, was studied. The vegetation, pH degree and transparency of the milieu were considered. In a table the distribution of the various mollusks in the different localities is checked. The author distinghuishes between two biocoenoses: 1. the Lymnaea ovata-coenosis in young pools, with turbid water, scanty vegetation and poor in mollusks, and 2. the Lymnaea stagnalis-coenosis in older pools, with clear water, luxuriant vegetation and much richer in mollusks. Finally the settling possibilities and the competition of the species in the different pools are discussed.