Two subspecies of Unio crassus Philipsson, 1788 (Bivalvia, Unionoidea, Unionidae) in The Netherlands

The former distribution of Unio crassus in The Netherlands is reconstructed on the basis of the collections of the Netherlands Centre for Biodiversity Naturalis, Leiden, and the private collection of the author. The most recent record of live U. crassus in The Netherlands dates from 1967, when two individuals were collected in the Biesbosch, in the southwestern part of The Netherlands. Arather common form of U. tumidus Philipsson, 1788, here referred to as forma curtus has often been confused with U. crassus and has caused considerable misunderstanding about the distribution of that species in the past. Next to the most common subspecies, U. crassus riparius, U. c. crassus is reported as a second subspecies from the northeastern part of The Netherlands. The conchological differences between the two subspecies are described and illustrated. Anew way of measuring, using the prodissoconch instead of the umbo, is introduced.