Bijdrage tot de kennis der Molluskenfauna van Oostvoorne

The molluscs of the island of Voorne were hitherto insufficiently known. Many years ago the present author had the opportunity to visit a great number of localities, giving more special attention to the freshwater species, as for landsnails the season was too dry. Various small ponds and lakes were studied and molluscs were abundantly collected. Although the number of species, Gastropods and Pelecypods, was not large and only about 24 species and varieties were found, various ponds could be compared and striking differences between them were noted. The author had special facilities to visit many localities usually strictly forbidden, because they are nature reserves where field study is but very rarely allowed. The largest lake, called „Breede Water”, a locality never studied malacologically, was visited twice and here a curious form of Lymnaea ovata Drap. was found with a very high spire and agreeing as to form and outline with the central european Lymnaea peregra Müll. Anatomically the true L. peregra Müll, is very different from L. ovata Drap., which is common in our country. The specimens found in the Breede Water agreed as to their anatomical characters, however, with L. ovata and represent certainly a variety of that species. This variety was identified as var. piniana Hazay, described from the danubial region in Hungary. This variety probably occurs in analogous localities in our coastal regions. Another curious lake called „De Waal” was explored. Here the water is more or less brackish, which was already evident as the surrounding flora contains a high percent of halophytic species. Potamopyrgus crystallinus carinatus was found there on odd valves of Cardium edule. The larger genera as Anodonta and Unio were not found in the regions of the dunes, but are abundantly represented outside these regions in ditches in the northern part of the island. Here the author found Planorbis laevis Alder in the socalled Heindijksloot. This species differs from Planorbis albus at once by its glossy aspect and wanting spiral striation. Being a very rare species in the Netherlands, it was found in our country only in a few localities, but in the year 1937 it was found on the North Sea Island of Texel. The collected material is in the author’s cabinet, the duplicates of most species are placed in the Rijksmuseum van Natuurlijke Historie at Leiden.