Acrothoracican cirripeds burrow in a variety of shelly materials dating back to the Carboniferous. The burrow is usually less than 5 mm in depth, with a smaller, tapered, slit-shaped aperture. Chemical action is proposed for initial penetration, and as an aid to the abrasive action of the teeth on the mantle and along the lateral bar. The burrowed shell is usually dead, and varies markedly in hardness