bgraves: I originally put it on that small rock, it attached, and hasn't moved since. It works well because I can move the small rock easily if the clam ever has to be relocated.
Mentat: For some reason, the clam just likes to sit that way. If I move the rock it's on, it just goes back to that same position. I can post some pics of my other clams when I have access to my work computer.