Can anyone identify these?

[Emma]

Well, you see, usually if they are an erchin they would be curved on the inside, not flat. What's more, I don't think they could be sea sponge because it's shape isn't so uniform. My best guess is that they are pieces of coral (maybe brain coral) and they might be flat on the other side due to wear and tear in the sea (like what happens in a river and makes stones smooth and round). This is my best guess! I'm sorry I can't help you further.

Best wishes,

Emma.