We present a history of the various collections of pelagic, or holoplanktic Mollusca, available in the Naturalis Biodiversity Center, Leiden The Netherlands. Both for the fossil and present-day collections numbers of available lots, stratigraphic and geographic origin and availability of type specimens are documented. We discuss four issues related to these collections: 1) preservation of pyritised fossils, 2) the backgrounds of the Maltese fossil collection material, 3) a list of all 505 taxa considered valid, 216 of which are currently living species (many of which also have a fossil record) and 4) diagnosis of Vaginellinae subfamily nov.