I need help...thats an understatement..

I need anID on a species of saltwater clam.When I bought this nano tank, it was an established setup.The live rock had small caves and in the caves were small ivory colored clams.The mantles are brown to chocolate in colors.They are about the size of a large snail shell.Live by filter feeding not by light.They are rather intriging, but I want to find out what they are so I can do an inventory of my tank.I also have a small clam that is colored like a freshwater clam, and a real pretty striped red clam(very small) that is attached to the underside(exposed) of a large live rock.Hard to get pics because they are in a dark spot.But they are alive and well and feed regularly judging by the opening and closing they do.
   Any help would be appreciated!
Thanks!

 

Fawn, thanks I'll look into that name.