Wikipedia
Prunum
Prunum is a genus of sea snails, marine gastropod mollusks in the family Marginellidae, the margin snails.
The separation between the genera Prunum and Volvarina is not very precise. They form together a monophyletic clade. The differences between these two genera are based on differences in the morphology of the shell, the radula, the radular cartilage, the mantle and the internal anatomy. A rather arbitrary criterion is currently used to distinguish between the two genera. The large species with a strong callus are placed in Prunum, the slender species with a thin callus in Volvarina, leaving in between many ambiguous species. To date (2010) there is no phylogenetic analysis behind the current generic placements