It is indeed a fossil, but a of somewhat different kind.
This is a trace fossil. In this case the animal was buried and the interior space filled with fine silty sediment. The shell was subsequently dissolved away by acidic groundwater. This left a cast of the internal space of the creatures shell.
The word for this type of fossil is "Stienkern" which I think is German for "Stone Clam".
As noted previously, it is a fossil of a clam (or bivalve). Your Stienkern is likly a few million years old. It looks like a Mercenaria mercenaria, commonly know on the East Coast as the Quohog or hard calm. The indians used the purple colored shells for a form of money - hence the latin name.
Paleontologically yours,
John DeMelas
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I am so not lesdysxic!
0.1 Creamsicle Cornsake "Yolanda"
1.0 Bairds Ratsnake "Steely Dan"
0.1 Desert Kingsnake "FATTY"
0.1 Black Rat "Roberta" RELEASED!!!