Some inarticulate Brachiopods attach one of their valves to a hard surface and lift the other valve like a cap. One of these oddballs is called Petrocrania and is commonly found on various shells within the Silica Formation. The shells are very plain with no real decoration so they sometimes look like a scab on the surface of a shell. Some even grow their shells to match the external ribs of their host shells.
This Petrocrania was in turn colonized by a Bryozoan (possibly Atactotoechus sp.). All you see on the underside is the interior of the top valve.