Paracyclas from the Silica Shale

I've only managed to find two genera of Pelecypod in my visits to the Silica Shale. The first on is Paracyclas. Below is a loose specimen that is a steinkern, or internal mold of both valves. It is not in the best of shape, It's color suggesting it was partially pyritized and it has now altered to Goethite.

Valve


Front


Valve


Rear


Profile



So how do I know this is a Paracyclas and not a Brachiopod? If you look at the first and third pictures of the valves you can see the "beak" is offset just a little. This is the hallmark of a Pelecypod, equal valves mirroring one another along the hinge line. Brachiopods mirror down the center of each valve and quite often have inequal valve sizes.


Here is another specimen with the internal mold still in matrix.