Bivalve Hitchhiker

[Cap'n]

I found a couple small clam-like bivalves on the live rock I recently purchased (thanks Deb).

They were attached pretty strongly to the rock. I figured they would prefer to be in the sandbed, so I pried them off and placed them at the bottom of the tank. One of them has situated itself with the open part of the shells upwards while the other promptly buried itself in the sand. They are about 3/4 of an inch long with white shells. The farthest I have seen the shell open is about 3mm. They have a dark, maroon to purple interior.

Has anyone had experience with anything similar?
Am I right in putting them in the sand bed?
How long can I expect them to survive?