Thin clam-like critter ID

[donny619]

Few weeks ago, I saw this critter and thought it was some 'debris'. During routine cleaning, I moved it behind my rocks only to see it slowly move it's way back up to this location. When I first saw it, it was thin and 'closed'.

Now I see it opening and closing like a clam but seems too thin to be one.

Can I get help with ID?

 

[Justfebreezeit]

Some species of scallop i'd guess.

 

[Crabs McJones]

Looks like a bivalve.