The life-histories of the two species of Cerithiopsis are described here for the first time, that of Triphora perversa is partly known already. All three species are common in Plymouth and their larvæ are very common in the plankton in spring and summer. Pelseneer (1926) has described the eggs and newly-hatched larvæ of Triphora perversa from the Brittany coast and Fischer (1884) has noted that it has a very long free-swimming life, still being a veliger with up to 7 or 8 whorls. The eggs of Cerithiopsis barleei have now been found at Plymouth and the larvæ reared as far as a stage just before settling down, the late stages from the plankton being reared to the crawling stage in plunger-jar. The eggs of Cerithiopsis tubercularis have not yet been seen, but the larvæ are common in the plankton and the late larvæ have also been reared in a plunger-jar until the crawling stage.