A pheasant snail (Eulithidium pulloides), grazing along a blade of surfgrass (Phyllospadix sp.). I first showed pictures of the shells of these snails in April 2014, but I haven't shown the entire animal yet. (They're hard to find because they're quite small — only a few millimeters long.)
One of the interesting characteristics of this species is its operculum (or "trapdoor"). In most marine snails, the operculum is made of a protein material (think about a moon snail operculum). But in some species, including these pheasant snails, the operculum is harder and made of calcium carbonate.
See below for a close-up of the white, calcareous operculum:
Can you think of reasons why it might be helpful for a snail to have a calcareous operculum? (One possibility is mentioned at the bottom of this post.)
If you're interested in finding a pheasant snail, try searching the bases of surfgrass clumps during a good low tide.
P.S. Calcified opercula are thought to be a defense against predators.