I've been finding some interesting clam fragments at Paulding, Ohio (Silica Fm., Middle Devonian) with original shell and a conspicuous sharp angle along the dorsal side of the shells. They are coming from the limestone layers of the formation (not the shale). Finally, I got one complete enough that I might be able to get an ID. It's about 3/4 complete, but unfortunately it's missing the anterior end, including the umbo. I've tried to illustrate the shape via some photos with reconstructions (red line) of what's missing. It's hard to illustrate the 3D qualities with photos. I'm thinking this is Mytilarca cordata. Looking for confirmation from those familiar with clams of Silica Formation or other Devonian formations of same age. The shell (what remains of it) is about 6cm in length.
@crinus