Found this while sorting matrix for micros.  It's shaped like a brachiopod or bivalve shell, strongly concavo-convex.  It has widely-spaced spines and very fine growth lines that follow the margin of the shell.  I found two of them, and they are about 4 - 5 mm in diameter.  My best guess is that they are "larval" shells of the gastropod Spiniplatyceras rarispina that have had the protoconch break off.  I can't think of any brachiopods from that locality and time period that would have spines lik