If you find a chunk of brittle rock with symmetrical holes, chances are it’s mudstone. Inside the holes you might find oblong-shaped clams called piddocks. The raspy, rounded end of their shell is used to grind out their burrows, where they spend their lives. Breaking the rock will often reveal the shell of the bivalve, completely intact. Living colonies can be found both intertidally and subtidally. These animals are edible, but collecting them requires chipping away the rocks, which destroys their habitat.

Mudstone holes also might contain peapod borers— long, slender, small brown clams that also drill their burrows.

Source: Flotsam, Jetsam and Wrack