Abstract
The spermatophores, the male head region and penis and the operculum of 10 species of Clithon and 14 species of Neritina are described. The female reproductive systems of species of Clithon and Neritina are compared. The study confirmed that two genera can be recognised. Species of Clithon, whether spined (e.g., C. spinosa, C. corona) or not (e.g., C. olivaceus, C. oualaniensis), have a curved indentation along the middle of the operculum, spermatophores with a long filament, a penis pouch in the head region and a short vaginal duct. Neritina species are more variable but their spermatophores lack a long filament, they have no penis pouch and they have a long vaginal duct that leaves the vagina near the spermatophore sac.