The first fossil is a right valve from a Modiomorpha concentra showing the typical elongated oval shape with fine concentric growth lines on the surface of the shell.
This species is somewhat common and I've blogged about them being found in the Mahantango Formation Here and Here.
The next shell is squished horizontally so it's hard for me to ID for sure. The thick, wavy concentric folds on the surface (which do not represent individual growth lines) could be from two closely related species: Grammysia bisulcata and Grammysioidea arcuata. Both look overall the same but Grammysia bisulcata has a "belt" that runs from the beak of the shell across the folds down to the margin where the valves meet while Grammysioidea arcuata does not.
Right now I'm leaning towards Grammysioidea arcuata since I can't see a recognizable "belt".
As further illustration of this species, here is a different specimen that I found on the same trip. This one has been compressed flat but at least one of the valves (the right valve) is intact.
I have collected G. arcuata before from the Moscow formation and also the Mahantango formation.
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