What is this round flat organism?

[Greengirl]

Can anyone ID this surprise in my Caulerpa tank? I have no corals but some snails and live rock and a small "mussel" or two. This is a different shape than my mussels, and in the right lighting I may be seeing an intestine within? It is on the bottom of the rock away from light.

 

[bushdoc]

It’s a mollusk. Looks almost like an translucent oyster. Some kind of limpet?

 

[Reefer Reboot]

Jingle shell (Anomia simplex).