As long as you have thoroughly washed and rinsed them, they are just calcium, secreted in layers, by the clam, as it grows.....you need to remove all flesh, including their hinge, that may still be attached to these shells.....you might want to check about the area(s) where you found the shells, to be sure there are no prohibitions/dangers to taking fish from those waters, if there is, the prohibitions could be from toxicity in that area, but if everything appears safe, you shouldn't have any problems.....boiling is always good for removing most toxins including oil residue.....
