50 collections were taken in the estuary of the river Dovey, N. Wales, throughout 1938 and early 1939, by means of a fine sieve. C. volutator is abundant below neap tide levels in the finer soils along the brackish streams that cross the estuary; C. arenarium is confined to coarse soils above the high tide level. In each collection of C. volutator, all individuals were measured for total length, sex detd. for each individual over 4 mm., and the sexual condition of the [female] recorded. Both mature and immature forms in the overwintering population measured 7 mm. in early 1938. The mature forms began to breed in Feb. and had died by Apr. The immature became mature from Mar. through July and had died by Aug. The young produced by this 1st generation became mature in July, and breeding continued until early Oct. This 2d generation continued growth through the autumn and winter to recommence breeding in March, 1939. The young produced by this generation also over-wintered. All [female][female] are mature at 6 mm., a few at 5 mm. The av. size of breeding [female][female] is 7 mm., a few individuals live to reach 10 mm. The broad lamellae develop gradually in all [female][female] over 4 mm. The hatching of a maximum number of young at some time each month from Apr. to Oct. suggests a lunar periodicity, but this could not be demonstrated with certainty. The av. size of the [male][male] was < the [female][female] with only a few reaching 8 mm.; [male][male] were always fewer than the [female][female], and the proportion decreased progressively from the 4- to the 8-mm. groups. The proportion of [male][male] is greater in early spring when the reproductive season is beginning and in midsummer when it is at its height. A few inter-sexes occur at all seasons.