But first things first. Sand dollars are highly modified SEA URCHINS that live on sandy bottoms, mostly in shallow tropical to temperate water places. Please make a note of it. (or go read this post here!)
Most sand dollars are either kind of disc-shaped such as this Pacific Dendraster excentricus.
But then you get THESE weirdos!
|This image is from Wikipedia!|
I should note that all members of the Rotulidae are also found as fossils...
1. The genus Rotuloidea. The first is this relatively simple looking guy.. Rotuloidea fimbriata. This species is only found as a fossil, occurring from the Miocene to the Pliocene (that's between 3-23 million years ago). Found in Morocco.
|Image from this Echinoids Gallery page|
|This image borrowed from this Excellent French site: Sciences de la Terre et de la Vie.|