Sand Dollar

 Sand dollars (san-doll-er) are flat, round marine animals related to sea urchins, sea stars, and other echinoderms.  The most common sand dollar, Echinarachnius parma, is a widespread in circumpolar ocean waters of the Northern Hemisphere, from the intertidal zone to considerable depths.  The ancestors of the sand dollars diverged from the other irregular echinoids, namely the cassiduloids, during the early Jurassic, with the first true sand dollar genus, Togocyamus, arising during the Paleocene.  The term sand dollar refers to the round flat shape of the test which is similar to a large coin.  By the time the test washes up on the beach, it is usually missing its velvety covering of minute spines and has a somewhat bleached and often slightly greenish appearance due to its exposure to sunlight.  Clypeaster rosaceus is a sand dollar that is much thicker than most sand dollars, and is often referred to as "sea biscuit".