Dendropoma annulatus:

Worm Shell. Members of this family, Vermetidae, have unspiraled shells which are plastered to rocks and old coral. This small species, which is usually less than a centimeter, can be very abundant in tide pools and on rocks in the lower intertidal area. The operculum, which usually appears as a black spot, is often more conspicuous than the shell. A larger species (D. irregularis), occurs in enormous numbers on the "boiler reefs" of the south shore.