Once a hard shelled animal dies it's shell often lies around on top of the muddy bottom until it gets buried. While it is exposed it provides a hard surface for many other encrusting animals that otherwise would not be able to establish themselves. Today I have two examples of trace fossils that were left on the interior surfaces of pelecypod shells from the Devonian aged Panther Mountain formation in NY.
First up is this shell which may be an Actinodesma erectum or another species. It's hard to say for sure because along the hingeline only the wing is visible and not the ear. But that is not why I kept the piece. What caught my eye are the spider like trails that criss cross the fossil.
Another pelecypod fossil from the same locality is shown below and this one has multiple, straight and wide traces on it.
To me these look very much like a boring clam or sponge that was digging a home within the shell of the expired clam.
Both specimens come from the Panther Mountain formation (Devonian, Givetian stage) at Cole Hill Rd. quarry in Madison county, NY.