While visiting Sanibel Island on a cold New Year's Day in 2011, I came across an arrangement of shells of the Stiff Pen Shell, Atrina rigida, on the beach, forming an ellipsis, with the year 2011 written with smaller shells.
I found many large, empty pen shells in the rich beach drift along the shore of Sanibel Island, especially on the southeastern part of the island.
Read more about this bivalve at the website of the Bailey-Matthews National Shell Museum, on Sanibel Island:
http://shellmuseum.org/shells/shelldetails.cfm?id=188