sand dollar

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.
2. n. The disklike internal skeleton of a sand dollar, having five radially symmetric oblong markings and often a pattern of slotlike holes.

Century Dictionary and Cyclopedia

1. n. A flat sea-urchin, as Echinarachnius parma, or Mellita quinquefora; a cake-urchin. The fishermen on the coast of Maine and New Brunswick sometimes prepare a marking-ink from sand-dollars, by rubbing off the spines and skin, and, after pulverizing, making the mass into a thin paste with water. See placenta, Scutellidæ, shield-urchin, and cuts under Encope, cake-urchin, and sea-urchin.

Wiktionary

1. n. 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

GNU Webster's 1913

1. n. any one of several species of small flat circular sea urchins, which live on sandy bottoms, especially Echinarachnius parma of the American coast.

WordNet 3.0

1. n. flattened disklike sea urchins that live on sandy bottoms