…the old Sand Dollar!
Otherwise known as the 5 holed keyhole urchin, these fellows are sometimes difficult to tell if they are living. When alive, they are covered in tiny spines that undulate, helping the urchin burrow. The ‘petals’ on the surface are gills, allowing water to circulate through.
Here’s a neat page on them: http://www.follybeach.com/seashells.php