Theodoxus and Neritina are distinguished from each other by their ontogeny (Bandel 2001). While the Theodoxus species hatch from the spawn as miniature adult, Nertina species leave their spawn as planktotrophic larva that will float in the sea for a more or less extended period before its metamorphosis to a crawling young. However, at the adult stage the taxonomic separation of species of the genera Theodoxusand Neritinais not always easy. As most of the Neritina spp. are marine species and usually have a denticulate border of the columella and two apophysis of the operculum, most species of the genus Theodoxus are limnic and have a smooth border of the columella and one apophysis (the “rib”); some also have a small apophysis, the peg, on the operculum (Glöer 2002). Further, in Neritina the peg is thick and strong, while in Theodoxus it is, if exists at all, small and weak. A revision of this family, particularly its subdivion in clearly defined genera is needed.