The successional mode of speciation of Julian Huxley (1942) may be defined as gradual transformation of one species into a successor species, or several of them, during the course of geologic time without primarily involving geographic, ecologic, or adaptive segregation. The purpose of this paper is to present a particular case of such a mode of speciation and to discuss problems involved therein. The example chosen is the stock of Cubitostrea sellaeformis (Conrad) [Mollusca, Pelecypoda], which contained 4 separate successional species of oysters and lived in Middle Eocene time in the ancient Gulf of Mexico.