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The presence of thin organic layers embedded in the nacreous layer in the shell of several species of Unionoida has been widely considered to be largely a reaction to prevent further shell dissolution once part of the shell has been corroded. Their microstructural characteristics have also been used as taxonomic characters to separate the families Margaritiferidae and Unionidae. This paper focuses on the description and interpretation of these organic layers in samples of four extant species of European Unionidae coming from acidic and alkaline waters, six extant species of Margaritiferidae, one Cretaceous Unionidae and two Cretaceous Margaritiferidae species. Our results demonstrate that organic layers have characterized species of Unionoidea at least since the Cretaceous and occur in specimens coming from both acidic and alkaline waters. Moreover, the presence of these layers in noncorroded shells can be interpreted as a preventative or ‘prophylactic’ measure against corrosion. Their thickness is the only useful character verified to separate the two families. In the Margaritiferidae, organic layers were found to be present in Margaritifera margaritifera, M. valdensis, M. idubedae, M. laosensis, M. laevis and M. monodonta, and absent in M. auricularia and M. marocana. In Unionidae, they appear in Protopleurobema numantina, Potomida littoralis and in some individuals of Unio mancus, U. delphinus and U. tumidiformis. We argue that organic layers played a fundamental role in shell protection during the invasion of freshwater by the earliest Unionoida, while today they function to improve fitness in acidic water in some species.

The Mediterranean trochid gastropods currently known as Monodonta mutabilis (Philippi, 1846) and Gibbula richardi (Payraudeau, 1826) are shown, on both morphological and electrophoretic grounds, to be sibling species, and therefore congeneric. The generic name Phorcus Risso, 1826 should be used for these species, which may be more closely related to the Mediterranean Trochocochlea species than to other species classified in Gibbula. The oldest appropriate genus-group name for the Mediterranean species currently classified as Monodonta (M. articulata, M. turbinata) is Trochocochlea MOrch, 1852. The earliest type species designation for Osilinus Philippi, 1847 is that of Trochus constrictus Lamarck, 1822 by Herrmannsen (1847), which would make Osilinus a valid senior synonym of the widely used name Austrocochlea Fischer, 1885 and not of Trochocochlea as commonly accepted. A ruling of the International Commission on Zoological Nomenclature will be sought for the case.