Vermetid snails are stationary. They don't look like your typical snails. The small one's have these purple to almost clear spirals that they live in, but the larger one's almost have a calcareous-like base, similar to your clam's shell.
Here's the best picture I could find of what I am thinking of. Don't know if it's similar to what you had but that's my best guess.
http://archive.reefcentral.com/forums/showthread.php?t=700825