In the estuaries in the South-West part of the Netherlands locally large accumulations of shells occur, which are exploited by a chalk factory at Den Briel. This factory obtains the shells at four different localities by means of a suction-dredger. The material from two of these localities has been studied and lists of species from each of the two have been included in the present paper. The first locality, a channel North of the most western part of the island of Goeree, yields nearly twenty species of Mollusca, of which Spisula subtruncata (Da C.) occurs abundantly. All these species but two are common or rather common members of the Dutch marine fauna. No elements of tertiary or quaternary faunas have been found between these shells; the accumulation must have been formed in recent or subrecent times. From this locality 10.000 m³ or more of shells have already been removed and there is no danger yet of it being exhausted. In the second locality, situated in the Westerschelde South of the island of Zuid-Beveland, the bank of shells in covered by about 6 m of sand. These shells are characterised by their pale-yellow or yellowish white colour. More than a hundred species of Mollusca have been found in the material from this locality, many of which are derivative from tertiary (eocene, miocene, pliocene) or quaternary (pleistocene, holocene) deposits. Of all these species Cardium edule edule L. occurs in the largest quantities. Some few very recent looking shells may casually have been mixed up with these shells. The author thinks it most probable that this accumulation has been formed in the holocene.