What are these?

[PaulMonfils]

These are opercula (singular operculum). An operculum is a sort of "door" that a snail uses to seal the opening of its shell when it withdraws inside. There are two different types. The commonest type are made of a semi-flexible protein called conchiolin, which is similar to the keratin which makes up animal nails and horns. These are usually just a dull brown color, though they come in a lot of interesting shapes. The other type, which you have here, are calcareous, made of the same substance as the shell. The flat side is attached to the snail's body, and the rounded side is visible, blocking the shell aperture when the snail is withdrawn inside. The ones you have are from turban shells. These are quite eroded, but when fresh the rounded side is often brightly colored and interestingly sculptured, or very smooth and shiny.

If you google "turban shells" you'll find quite a few pictures of various species with the operculum in place, as well as some pictures of bunches of opercula, which are often used in making jewelry or other arts and crafts.