Some type of bivalve

[gtguy256]

So I removed my torch to treat an infection and found this thing hiding. I had seen it before and thought it was dead but it moves when I shine a light on it.
Any ideas what it is?

 

[DaJMasta]

Jewel box clam?

 

[ISpeakForTheSeas]

I can only see one of the shell's clearly, but (from the fluting) I'd guess something like a Cockle or Ark Shell - can't tell for sure from the pics though (and bivalve ID can be hard even with super clear pics).