In April of this year Mr. J. P r i n s drew my attention to a locality in the neighbourhood of Vreeland (prov. Utrecht) where Hydrobia steinii occurs in great quantities. I availed myself of this opportunity to study this little snail more closely. The different authors do not agree about the systematic position of this species within the family Hydrobiidae. At present the view prevails that it is no Hydrobia. C 1 e s s i n was the first to place the present species in another genus, in his """"Deutsche Excursions-Mollusken-Fauna"""" we find it in the genus Bythinella (4 x), p. 326). Several authors have followed C 1 e s s i n in regarding Hydrobia steinii as belonging to Bythinella and recently K r u 11, who had studied the anatomy of Hydrobia steinii, arrived at the conclusion that it is really a true Bythinella (11, p. 462, nr. 15).