any of an order (Clypeasteroida) of flat, round, disklike echinoid echinoderms that live on sandy ocean beds
See sand dollar in American Heritage Dictionary 4
noun
Any of various thin circular echinoderms of the class Echinoidea, especially Echinarachnius parma, of coastal northern Atlantic and Pacific waters, having a covering of short movable spines.
The disklike internal skeleton of a sand dollar, having five radially symmetric oblong markings and often a pattern of slotlike holes.