Sir Charles W. Shults III, in his A Fossil Hunter's Guide to Mars, has unearthed (unmarsed?) several photos taken by the Spirit rover of objects on the surface of Mars that look very much like terrestrial seashells. One of them is shown here compared to an Earth seashell (inset). It appears to have a thin structure and a spiraling shape. If they are rocks, they are mighty peculiar rocks. You can see more here.