Thanks to all of you who jumped on this topic.  You've given it a good ride.
I don't see the shell can be a Pyganodon, the beaks are too low and it's more elongated than seems right for Pyganodon.  The same goes for A. couperiana.  It really looks like U. imbecillis, no matter how funny the name.  And, yes, it is a thin shell even at 153 mm long.
Paul Parmallee (The Freshwater Mussels of Tennessee) shows U. imbecillis well into PA and NJ as well as the Virginias.

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