Crocea clam

[Delphinus]

I prefer rocks myself but many people do keep them in the sand, so whatever works best, but in general: 1) closer to the light is better (usually) with crocea's 2) they're found inside rock in the wild (like Andestang mentioned) .. it's pretty cool, all you see is the mantle .. and 3) less chance of some kind of opportunistic critter getting inside of them via the byssal opening if they're on the rock (no guarantees of course, but sand is easier to travel through than rock).

If they don't stay ON the rocks though it's better just to let them be where-ever they prefer to be. Sometimes that's the sand.