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looks to me like some form of brachiopod (not bivalve as the different valves are not equal) and the small hole you see at the top of the specimen is the pedicle or foot, a muscle would extend out to connect it to the seabed

very cool specimen

also if you have a look on image 2 there is a second brachipod half revealed if you fancied popping it out, should be pretty good as well

unfortunately I have no idea what brachiopod it is, if you google the geology of the area you found it, it should say how old the rocks are then if you google brachiopods that were alive at the time then it should be pretty easy to distinguish which one it is as they often have distinguished features

sorry can’t be of much more help only a geology student at the moment, but any further enquires give me an @ and I will do my best

Here in Springdale Arkansas the fossils are from 340 to 400 million years

so most probably carboniferous, I’ll have a look for you

due to the shells I’m struggling to find an exact idea, as the ribs of the shell have been rounded of during fluvial movements before fossilization, however it could be a relative of athyris or composista

however,I’m afraid I can’t be of exact help, very hard to identify specimens without having them in your hand

I just posted a couple more pictures of it, the end Had a little white horn like peace to it that has been unfortunately lost. Looked like a beak

Thank you so much Ben

Nice brachiopod steinkern, or internal mold!