Large pen shells are common on Sarasota's beaches. They are often broken when they wash up and covered by barnacles or algae.
Pen shells average 8” in length and live in soft, sandy mud with their narrow tips downward. A small foot spins clumps of tough threads that keep it attached to buried stones and broken shells.The silky threads are called "byssus".
Look closely at the wide end of the shell. The short, prickly spines on the wide end characterize "rigid" pen shells.