UCMP MYSTERY FOSSIL #45 is . . .
the cystoid Cheirocrinus cf. forbesi

Cystoids were echinoderms, first appearing in the middle Cambrian and going extinct in the Upper Devonian. They were most numerous during the Ordovician. Cystoids resembled, and lived alongside of, the more familiar crinoids—it is possible that cystoids went extinct because they were outcompeted by the more efficient crinoid filter feeders. The UCMP website has a gap in its information on this particular group of echinoderms (and there seems to be very little information about cystoids anywhere on the web) but you can read more about echinoderms in general here.