Any shell collection is liable to have a few sand dollars. They're not mollusks, but they're fun. The immense variety of life on earth is such a miracle. So is its age! These specimens were collected only a few miles apart. and there could be 50 million years between them. These slightly thicker versions are often called sea biscuits. The rock might be oil shale, but I'm no geologist. And to tell a tale out of school, neither is my wife. Geology is the only class she ever failed (yes, that lab exam does seem to have been less than fair). That was the summer she learned to play bridge. So oil shale or not, we don't know!