What are the different types of sand dollars?

Sand dollars are part of the class Echinoidea and primarily belong to the family Clypeasteridae. The most commonly recognized types include the common sand dollar (Echinarachnius parma), found in shallow coastal waters, and the keyhole sand dollar (Dendraster excentricus), which has distinctive holes on its surface. Other notable varieties include the Pacific sand dollar and the Atlantic sand dollar, each adapted to their specific marine environments. These species vary in color, size, and habitat preferences, but all share the characteristic flat, disc-like shape.