Yeah they have a foot that grabs onto the rock when they find their happy spot. One thing about Crocea clams is they have to have intense lighting, so they may have been moving to try and find some appropriate lighting. One other thing is they don't seem to like too high of a flow. If it's mantle was flying around then it might not have liked that either.
Before it died, was the mantle coming out fully? Was there any pinched mantle issues? What are your water parameters?
I find it hard that they both just up and died one day and there is nothing left in the shell. There should be some stinky remnants of the clam in the shell....
What other inhabitants have you gotten in the tank?
When I first got mine, I put it on the sand with a shell under it as I wanted it in the sand bed as well, however it kept moving around the sand bed everyday looking for a better spot. I then moved it up onto the rocks closer to the lights and it seemed to latch on and be happy. I had a different lighting system than my radions now, so I am sure that I could put it in the sand with a rock or shell under it and it will attach.