A common Pelecypod found in the Mahantango formation is Leiopteria. As is often the case, the example below is of an internal cast and external mold found recently.
This is how the piece was found lying on the surface. You can see the internal cast resting within the outline of the external shell mold.
Here I have removed the internal cast. All the loose, brown material is from small bits of the sediment that were trapped within the shell. Once the shell material dissolved away they were left behind. This is a common occurrence and tends to lend a brown color to your fingers once handled. Note the extended hinge line visible on the upper right. This anatomical feature helps place the animal in the same family as modern day Pearl Oysters.
These last two pictures are of the internal cast. You can see some internal shell details preserved such as the pimple like muscle attachment points seen near the tip.