On the taxonomy and variability of Recent European and North African marine species of the subgenus Rissostomia Sars, 1878, of the genus Rissoa Desmarest, 1814 (Mollusca, Gastropoda, Prosobranchia)

From an extensive conchological investigation it is concluded that the Recent forms of the subgenus Rissostomia Sars, 1878, probably belong to three species only. Of these, Rissoa labiosa (Montagu, 1803) is represented by a great number of local forms from the Black Sea to the North Sea and the Baltic. Shells of R. membranacea (J. Adams, 1800) and of R. paradoxa (Monterosato, 1884) can often only be separated from those of R. labiosa by the larger apical dimensions. R. membranacea occurs in many local forms in north and north-west Europe. It seems to have its southern limit in south Bretagne, France. R. paradoxa seems to be restricted to Tunisia. A neotype of R. membranacea and lectotypes of R. labiosa, of R. paradoxa and of R. tunetana Pallary, 1912, a junior synonym of R. paradoxa, have been selected.