Sand Dollars are not technically shells, but they are still very cool!
The science bit: A sand dollar is technically a sea urchin, flattened dorso-ventrally. It is a type of echinoderm, which is a group that also includes sea urchins and “starfish”…. which, of course, are not “fish” at all. Like all echinoderms, the “keyhole urchin” or “sand dollar” has radial symmetry with 5 matching parts instead of bilateral symmetry with two. In life, the sand dollar lives on or just under a sandy bottom, typically below the low-tide mark. It has tube feet and spines (often gone by the time a dead one washes up on shore) and creeps along as it feeds on diatoms, algae detritus, and small crustaceans such as the ubiquitous copepods.
If you find yourself out on a sand bar and feel a sand dollar below your feet, go ahead and pick it up, if it is dark in color and looks a little fuzzy, let it crawl along your hand, but put it back. If you want to take a sand dollar home, stick with the bleached ones on the beach or your car, house, life, dreams, etc. will stink terribly.