The calcitic columnar prismatic (CCP) microstructure constitutes the external shell layer of pterioid and ostreoid bivalves. It consists of calcite monocrystals, which extend perpendicular to the growth surface and are surrounded by thick interprismatic membranes (<1-8 microm). The exceptional biomechanical properties of CCP layers make them interesting for biomechanical-biomimetic studies. Their potential as biomedical implants still requires further testing. Previous studies attributed the mineral phase a crucial role in the organization of the microstructure. Conversely, our observations on the morphology of prisms and abnormally calcified shells reveal that the organic web may form in the absence of the mineral infilling and that the boundaries between prismatic units are shaped by interfacial tension. Accordingly, we consider interprismatic organic membranes as the main shaping agent. We propose that the prismatic layer initiates as an emulsion formed by the fluid precursor of organic membranes and the extrapallial fluid, which causes formation of empty organic cavities. Prismatic crystals began to form within the organic cavities a little later, but their secretion surface soon levelled that of organic membranes.