UCMP MYSTERY FOSSIL #41 is . . .
the helicoplacoid Helicoplacus

The helicoplacoids are one of the oldest groups of echinoderms to appear in the fossil record. Shaped something like a fat cigar, they are thought to have lived upright, either in burrows or with a portion of their bodies partly buried in the substrate. Complete fossils of helicoplacoids have been found only in the White Mountains of California. Find out more about helicoplacoids here.