I found this little half-shell in SE WI. I think it is a brachiopod, but am not entirely sure.
Originally it was more thickly encrusted, but I've given it a vinegar bath for about 2 days, and more details have now become visible. It is currently back in fresh vinegar, in the hope more of the crusty layers will dissolve.
I would appreciate your help with identifying the shell and also its interior visible parts if possible. For example, none of the anatomy drawings shows interior "separation walls" such as I see here. And specifically, what is that thick appendix sticking out the side of the shell? Its end looks like fossilized soft tissue to me. Is this the pedicle? If not, what is it? If yes, I thought soft tissue barely ever gets preserved?
Interior
Side view