Marine molluscs new for The Netherlands, washed ashore at the beaches of Ouddorp (Goeree-Overflakkee, Province of Zuid-Holland), with some remarks on the occurrence of Altenaeum dawsoni (Jeffreys, 1864)

An investigation of small shells from the beach of Ouddorp on the island of Goeree-Overflakkee has yielded a number of species new to The Netherlands. Twenty-five species are enumerated and occasionally discussed in detail. According to potential derivation the species may be divided into four groups. These are (1) Lusitanian species (species having recently migrated from the south, nos. 2, 5-8, 11-15, 20, 21, possibly also 1 and 10), (2) Boreal species (species having migrated southward during a cold period, nos. 3, 18, 23, possibly also 1), (3) species which reached our coast as free-swimming larvae (possibly nos. 10 and 18), (4) Eocene species reworked from marine deposits (nos. 4, 9, 16, 17, 19, 22, 24, 25). Altenaeum dawsoni (no. 23) is considered to belong to group 2; this is in contrast to Spaink (1973) and Van Dalsum (1986), who hold the opinion that the species had migrated northward into the North Sea.