This is the operculum from a large Moon Snail. We found it on the beach in St. Augustine, Florida. The operculum acts as a door for the shell. When the snail is threatened, it pulls inside, and uses the operculum to block the entrance. As the snail grows, it adds to the operculum just as it adds to its shell. That way, the "door" always fits tightly.

Not all snails have an operculum, and they vary tremendously in their color and texture, but Moon Snail opercula are some of my favorites. My wife Nancy used this one in one of her art pieces.