Hercynella is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of fossil bivalves of late Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 or (more commonly) Early Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 age, found in Europe, North America, western Asia, North Africa and Australia. The name was also invalidly applied to a genus of moths now known as Tulaya

Tulaya

Tulaya is a genus of moths of the Crambidae family. It was previously named Hercynella but that name was pre-occupied by a genus of fossil molluscs.-Species:*Tulaya margelana...

.

Species were originally thought to be patellid

Patellogastropoda

Patellogastropoda, common name true limpets and historically called Docoglossa, is a major phylogenetic group of marine gastropods, seen either as a clade or as a taxonomic order.-Taxonomy:Patellogastropoda was proposed by David R...

 gastropods and it was not until 1950 that H. & G. Termier realised that they were bivalves. Prantl (1959) showed that both left and right valves could occur in two forms, which had in the past led to palaeontologists erecting two species where there was only one.

The best review of the genus is Forney et al. (1981).