The Common slipper shell, Crepidula fornicata, a.k.a. boat shell, a marine gastropod, or snail, pilled up at the Jetties on Nantucket. A not particularly rocky area, the island’s surrounding waters present less than enough bases for these snails to attach onto, so they often attach to each other, in chains. The species name comes from the Latin fornix, meaning arched chamber, the same root as fornication.
The gulls think they’re delish.
An update 1/15/11: Many snail species are hermaphrodite. Crepidula species start out as male and then develop female characteristics as they mature. Also, the empty shells, a.k.a. Quarterdecks, are used commercially as a bedding for embryo oysters.