Figure 1 illustrates the activity patterns of Clithon faba and C. oualaniensis during 24h. The two species were most dense on the substratum right after the tide receded, regardless of day and night. The number of exposed snails decreased subsequently, and was least while the water covered the substratum. Their number under water was larger at night than in the daytime. The percentage of moving snails changed approximately in parallel to the number of snails exposed on substratum. The number of snails buried partly in sand was relatively large on rising or receding tides, in consistent with the field observation that the snails were on the way of burrowing into or crawling out of the sand on these occasions. The percentages of pairing snails were larger than 20% through low-tide periods in most cases in each species. In a supplemental observation around the study site at the daytime low tide on July 30, 1989, pairs in the two species were formed almost exclusively between conspecific individuals, even at sites where the two species were intermingled. In one case of C. oualaniensis, an elongated spermatophore was seen on the way of transport between the pairing snails when they were detached. Therefore, the pair formation in C. faba and C. oualaniensis seems to be, at least partly, a sexual contact within each species.