Clypeasteroida (sand-dollars; subphylum Echinozoa, class Echinoidea) An order of irregular echinoids with ambulacra that are wider than the interambulacra on the lower surface. Accessory Tube feet are developed outside the ambulacral petals. The order first appeared in the Palaeocene.

sand dol·lar • n. a flattened sea urchin (order Clypeasteroida) that lives partly buried in sand, feeding on detritus. Several genera and species include the common sand dollar (Echinarachnius parma) found mostly along the coasts of North America and Japan.