ID this ?crab? ?Mollusk?

[Azial]

I bought a real nice colony yesterday, and when I had done accimating it I placed it in the tank. I noticed one piece of the "rock" part of the polyps was lose so I jiggled it hoping to just remove it. It came off and looked like a clam. I placed it next to the polyps and turned on the lights to get a pic of it:

I apologize for the blurriness it was the best I could get but you can make out the bisected shell.

http://img406.imageshack.us/img406/3723/1002026gn7.th.jpg (http://img406.imageshack.us/my.php?image=1002026gn7.jpg)

http://img257.imageshack.us/img257/8537/1002023wg5.th.jpg (http://img257.imageshack.us/my.php?image=1002023wg5.jpg)

http://img241.imageshack.us/img241/4425/1002026uq9.th.jpg (http://img241.imageshack.us/my.php?image=1002026uq9.jpg)

So I contacted the company I got it from and they advised me that it was an empty shell that was embedded in the live rock. Not a bad thought and I can certainly understand their point of view. However before night (and after I contacted them) I moved it onto the sand bed away from the polyp.

Well this morning Franken Clam moved back to it's Polyp, and my concern is that it's consuming it. I know most clams are filter feeders, but since I can't seem to identify it, it's hard for me to see what it is:

http://img87.imageshack.us/img87/571/1002028tg7.th.jpg (http://img87.imageshack.us/my.php?image=1002028tg7.jpg)

http://img81.imageshack.us/img81/3883/1002027qt2.th.jpg (http://img81.imageshack.us/my.php?image=1002027qt2.jpg)

any ideas? I am sorry for the blurriness these were the best of the sets.

[LeslieH]

Bivalves move by extending the foot & dragging the shell across the sand or rock. Live ones are often found in live rock & among corals. Yours is just moving around to find the best spot for it's filter feeding. They're completely harmless.