This study is intended to draw attention to the potential diversity at the species-level in the genus Hemidiscus, both fossil and Recent, to reject unsupported explanations of polymorphism for ‘variable’ sets of specimens, and to act as a prolegomena for its eventual re-assessment, along with its relationships within the family Hemidiscaceae. It is not intended as a revision of the genus nor is it a complete evaluation of the species-level diversity. It is a summary of some of the types found in the BM, especially those still in the genus Euodia, to provide arguments for not sinking all species currently known in Hemidiscus into Actinocyclus. Three new species are described: Hemidiscus kociolekii, Hemidiscus moronensis and Hemidiscus gigantea, the latter two are new species solely because they lack a validly published name. Four new combinations are proposed.