On the occurrence of Clausilia dubia (Gastropoda, Pulmonata) in north-western France

In 1957 I discovered a flourishing colony of the clausiliid lane snail Clausilia dubia Draparnaud (identification checked by Dr. F.E. Loosjes, Wageningen) on the seaward face of the coastal cliffs near Cap Gris Nez (Departement Pas de Calais) in north-western France. As this locality is far outside the known range of the species, further observations were carried out during short visits in 1960, 1964 and 1966, of which the results are summarized below. Cap Gris Nez, which marks the traditional boundary between the North Sea and the Straits of Dover, is a 50 m high promontory of a Portlandian sandstone, locally known as ‘Gres de la Crêche’, in the region called ‘le Boulonnais’. Geologically this region is the easternmost part of the Weald-anticline, to which the British shores of the Straits of Dover also belong.