An unbelievable in-situ, perched on a log gloriously at the base of a waterfall. Sat there for at least half an hour, probably longer. We found one other individual nearby, as well as an expected egg mass, which is in the last photo

Marion County

Looks like a butter clam, with some sort of mutation to the shell. I've only ever seen this once in person, but oddly enough there is another observation of another butter clam shell with the same mutation, found in the same place. Maybe a local environmental factor causing irregular growth patterns?