Disc shaped invert?

[kweigum]

Curious what this disc shaped invert might be on the sand? Below and right of the feather duster. Moved a couple rocks yesterday and found it underneath the sand it’s flat 1/8-1/4” and the under side is a dark brown about a 3/4” diameter

 

[KrisReef]

That’s a snails cellar door.

 

[IKD]

Agree, dead snail part

 

[kweigum]

And that’s why the underside looked like a snail lol. Should have been my first thought . Thanks