Modiomorpha is a Pelecypod from the Devonian aged, Mahantango formation of Central Pennsylvania. It's not too uncommon but getting a nice specimen can be tough as the dominant fossiliferous rock is shale/mudstone/siltstone. Below is a nice cast of the outside of one valve that shows the typical form: Oval shaped with fine ribs spaced evenly.
In the Mahantango formation it is common to find just molds and casts of the exterior and interior of shells. The rock is so porous that ground water dissolves the calcite shells long before we find the fossil. This next specimen is very interesting to me as it preserves the inside and outside of the shell in the same specimen.
The last photo above is the mold of the inside of the shell. When you look closely you can see the scar where the muscle that closed the shell was anchored.
This is the internal mold set back into the rock and it fits perfectly over the valve cast.
Here is how the rock looks when you put both halves back together without the mold.
And now with the mold inserted.
Pieces like this get me excited as it makes the animal more real to me. One day I hope to find a whole clam preserved like then and maybe I can make a replica from the casts and molds.