About 1/2" across, found on giant kelp.
San Diego County, California, US
Here's the only shell of a Crassadoma gigantea (Purple-hinged or Giant Rock Scallop) I've ever found. It was about 4" across, and the home for an Aggregating Anemone.
about 1" in diameter
About 1" across. Interesting article from about a year ago: http://www.sfgate.com/science/article/2-million-oysters-in-bay-begin-restoration-effort-4984300.php