Bivalved Gastropods
The great majority of shelled gastropods or snails have a shell in one part, hence the older name "univalve". The gastropod operculum, when present, even when it is composed mostly of calcium carbonate, is not considered to be a valve.
In contrast, species within one family of small sea snails, the Juliidae, opisthobranch gastropod molluscs, have a hinged shell which is composed of two parts joined by a ligament. These two parts very closely resemble the two valves of a bivalve. This group of species are often referred to as "bivalved gastropods". These are sacoglossans in several genera including Julia, Berthelinia, Midorigai, Edenttellina, Tamanovalva, and Candinida.
These bivalve gastropods were for a long time known only from fossils and dead material. Because of this, they had been described as being somewhat atypical bivalves. It was not until living individuals were found in the late 20th century that it was understood that these are in fact very unusual gastropods.
Read more about this topic: Valve (mollusc)