Abstract
The shell and opercular structures of Nerita (Theliostyla) albicilla and Cinnalepeta pulchella were described and compared with those of other extant members of Neritopsina. The shell of N. (T.) albicilla is composed of four layers : the outermost prismatic layer, followed by the simple crossed-lamellar layer, the myostracum, and the inner complex crossed-lamellar layers. The operculum consists of three prismatic layers deposited on both sides of an organic layer. C. pulchella also has a four-layered shell, but lacks an operculum. The outer layer is a homologous structure. The shells of Recent neritopsine families can be categorized into a four-layered group (Neritiliidae, Neritidae, and Phenacolepadidae) and a three-layered group (other families). In contrast, opercular structure is markedly variable in the Neritopsina, and little correlation can be established in the light of phylogenetic evolution or adaptation.
