White piddocks dig themselves into wood, blocks of peat and even soft stone. They are even sometimes found alive is such washed up material. They don't eat the wood, but filter out the plankton in the seawater. The shell is thin and fragile. Horizontal and vertical ribs run across both valves and small spines are found at the cross-point. This shell has such a different appearance than other shells, so you're not likely to confuse it should you find one.