A recent trip up to Arkona netted me a wealth of new genera from the site that I didn't have before. One of them was this Paracyclas that was found in the Arkona formation.
It's a steinkern (a mold of the inside of the shell) that has been completely replaced by Pyrite which is now altering to Goethite. This is the largest example I've found at this locality. I have another but it's much smaller, about 1-2mm in diameter.