In a new synopsis of the family Limapontiidæ the differences between Limapontia capitata and L. depressa, which may sometimes be mistaken for one another, are specially noted. These two species and Acteonia cocksi, the remaining member of the family, are sap-suckers that feed on the cell contents of green filamentous algæ. The cells are slit open one at a time by the leading tooth of the radula and their contents drained. The host plants of A. cocksi and L. capitata are marine Cladophora spp.; L. depressa feeds on Vaucheria. The size of the buccal mass varies according to the host plants. The subterminal position of the anus in L. depressa is an adaptive feature. The reproductive system of A. cocksi is described and compared with those of the other two species. During copulation both A. cocksi and L. capitata transfer sperm by hypodermic impregnation: sperm are injected by means of a penial style into a closed bursa copulatrix. The bursa of L. depressa opens to the exterior by a short duct and the penial style functions as a conducting pipe. Reasons are given for suppressing the genus Acteonia, Quatrefages, and including A. cocksi in the genus Limapontia, Johnston.