Any idea what this is?

[Badgers reef]

So in my mates tank he hasnt got access to the internet and im unable to tell him what it is can anyone help...
Its attached to a piece of live rock closes and opens slightly and when it opens slightly it seems to have something coming out of it like tenticals maybe?? Its like two flat pieces of rock really thin but pretty much binded together

 

[Zasman21]

What are we supposed to be looking at?

 

[Ricksreef]

Looks to be an oyster.