Here are a couple of interesting facts on sand dollars:
Sand dollars are usually crowded together over an area—as many as 625 sand dollars can live in one square yard (.85 sq m).
It seems there are at least four or five different types of sand dollars or sea urchin, including the common sea urchin (Echinarachnius parma) in Xanadian coastal waters.
Note - this is just a little info I found on the web, nothing I was really familiar with until I did a Google search.
Edit: A little more info: Sand dollars are found in temperate and tropical oceans throughout the world; usually found in shallow, sandy areas. They are from a class of marine animals known as Echinoderms, which literally means "spiny skin." They are closely related to sea cucumbers, sea urchins, and sea stars. In British Columbia there is one species of sand dollar called the Dendraster excentricus.