Thanks again. I believe they're a mixture of clams, oysters, and mussels. I'm not sure, but I think I've found a few scallops too.
The tank is only a 55gal, but everyone that sees it says it looks bigger.
Since these pics I've added 2 small bubble stones under the bottom rock in the middle. I think it looks pretty cool myself. I'll have to take some new pics when I get back into town on Monday.