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These shells (see in green circle) are empty inside which look like very flat marble donuts/coins. The small ones are like dollar coins; big ones can be cup coaster-sized.

It has beautiful five-fold symmetry (see the top one) on its back and a whole on its ventral part. Must be related to sea urchin?

What's more interesting is that some of them (see the one at the bottom) have a tiny shell of a different kind anchored on them.

enter image description here enter image description here

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  • $\begingroup$ In your second photograph, are you referring to the white conical shell or the little black thing? $\endgroup$ Jan 6, 2023 at 15:14
  • $\begingroup$ sorry for not being clear. Trying to refer to the same thing - flat white shells $\endgroup$
    – Code42
    Jan 7, 2023 at 14:33

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They are definitely sea urchins, specifically a type of sand dollar called the Eccentric Sand Dollar, which is found all along the Pacific coast of of the US. Here's what they look like when they're alive:

enter image description here

They're filter feeders partially buried in the sand.

If you're referring to the white, conical shell attached to the sand dollar, that's a barnacle, probably a California Barnacle, but the species is easier to identify alive (or newly dead) than bleached and eroded (to me, anyway.)

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  • $\begingroup$ you are amazing! $\endgroup$
    – Code42
    Jan 7, 2023 at 14:35
  • $\begingroup$ @Code42 - Thanks, but I just happen to like sand dollars and I'm good at searches. :) $\endgroup$ Jan 7, 2023 at 16:26

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