small black bivalves?

Burrowed into my live rock I've found several small, black bivalves.  They are about the size of a dime and have straight shell edges.  They seem to move around a bit since I just found another one perched atop a rock for the first time.  Otherwise, I've seen them burrowed deep into the rock.  Just wondering what they are - scallops, mussels, clams, etc?  Tank has been set up for over 3 months now and I've not fed them anything extra (phyto, etc) but they are still alive.  I'm wondering if I should feed them or if they will live or if they are bad or if there are any good reference web sites that could possibly help me ID them.