sand dollar

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various thin circular echinoderms of the order Clypeasteroida, especially Echinarachnius parma, of coastal northern Atlantic and Pacific waters, having a covering of short movable spines.
• noun The disklike internal skeleton of a sand dollar, having five radially symmetric oblong markings and often a pattern of long narrow holes.

from The Century Dictionary.

• noun A flat sea-urchin, as Echinarachnius parma, or Mellita quinquefora; a cake-urchin.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An echinoderm, of the order Clypeastroida, that has a flat, disk-shaped body with the mouth in a mid-ventral position, and lives in sand, on or near the surface

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun flattened disklike sea urchins that live on sandy bottoms