Around my pond, I have 'mussel-clam' type shells also.

Some are 5-6" long. Not sure what specific species they are.

I'm thinking it's racoons that dig them up and eat them.

I think it is also a sign of clean water.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."