INTRODUCTION Engaged in the study of fossil shells belonging to Thiele's sectio Cerithideopsilla of the genus Cerithidea Swainson and deriving from plio-pleistocene beds in East Java, I failed to find a satisfying account on the recent species of this group. Therefore I resolved to examine as large a material as possible myself in order to settle the value of the different species and varieties that have been described in literature. This investigation led me to the conclusion that even after the inspection of some 1950 specimens I cannot speak the final word, as ecological observations seem necessary to test the taxonomic value of the characters used for distinction. But a preliminary order may be established by the following revision of the material available to me, in which I propose to distinguish four species until field observations have thrown more light on the variability of these molluscs 1). Tryon (1887, pp. 159/160)—with his usual taste for lumping—cites the following synonyms of "Potamides fluviatilis Potiez et Michaud" in his Manual of Conchology: Cerithium microptera Kiener, Cerithium alatum Philippi, Cerithium radix Dufo, Cerithium incisum Hombron et Jacquinot, Cerithium retiferum G. B. Sowerby 2 and Tympanotonos euryptera A. Adams M. S., G. B. Sowerby 2. Judging from the description (Dufo, 1840, p. 62) Dufo's radix seems to be a true Cerithium. My material contained no specimens matching the figure and description of Cerithium retiferum G. B. Sowerby 2 (1855, p. 892, pl. 186 fig. 295) and therefore I shall not discuss this form, which at any rate seems closely related to Cerithidea cingulata (Gmel.). The remaining forms