Still going through my flats from my 2010 field collections in the Mahantango.
The fossils below are what I believe are Grammysioidea sp. pelecypod. It is smaller than it's cousin Grammysia and also lacks the longitudinal groove that is the telltale mark of that genera. As you can see the shell is slightly distorted. This is common in parts of the Mahantango formation and is likely due to post burial deformation by burrowing worms or trilobites.
These next two are the same shell but both valves are preserved.
Another complete shell.