Etching with the glutaraldehyde-acetic acid solution reveals that the nacreous tablets in Mytilus, Nucula, and Unio are composed of four crystal individuals which occur in two structurally different pairs and which are probably cyclically twinned. In Mytilus, one pair consists of horizontal lamellae, whereas the other pair lacks the lamellae. The lamellar pair of crystals is considerably less soluble in the etching solution than the non-lamellar pair. In the less-soluble crystal pair of Nucula and Unio, the lamellar structure is absent. In all three genera, the nucleation of new nacreous tablets invariably takes place on the top surface of the less-soluble crystal individuals. The stack-like growth of the nacreous tablets in the marginal region of the shell in Nucula is probably related to the exceptionally large-sized, less-soluble crystal individuals in the nacreous tablets of this region.