Kuli, you won't believe this story,

Today I went to my LFS to pick up some more LR for my display, and while I was picking through the rocks I was thinking about your clam...and then I look and I see a rock that looks like a huge clam. It is the classic clam shape and it had a seem running all the way around. So I buy the rock for $3 and race home and put it in my tank. At first I thought maybe I was wrong, and it was just a rock, but about half an hour later, it started to open up and I took this pic.

I don't know if anyone can ID it, but I would sure like to know what it is.

My bumblebee snail was nice enough to pose on top of it, so you can get a size comparison. It weighed 3/4 of a lb.