ID please

[dbelli]

Bought this at my lfs as a piece of live rock. just looked interesting. after putting it into my tank and letting it settle in I noticed something strange. This piece of rock was splitting open in the middle and was opening. It has 2 openings like a clam. I am assuming it is some sort of bivalve. any ideas?
(sorry, not the best picture)

 

[m0nk]

It's really hard to make out anything in the picture.

 

[dbelli]

okay, so i circled a couple of things in the picture. first, the big circle shows the whole "rock". the smaller circle shows the opening of it. Any ideas?