What made this beach button?

The tide was at its lowest; we'd walked and waded to the end of the shallows off Centennial Beach, and were walking back to shore. On the first sand bank, that one just ahead ...

... we saw this:

It was sitting like a bead on the sand, perfectly firm and rounded, about an inch high and wide, with a large hole in the center top. There was one other several metres away, but no more.


I dug my little shovel in beside it, one quick jab a good four inches down, and turned over the sand. There was nothing in it. Just sand. It didn't even squirt water at me.

There are some little castings beside the mound, and along the troughs of the nearby sand ripples. Those whitish things seem to be small snail shells. We didn't even notice them at the time, in our hurry to dig up whatever it was and get going; the tide was coming in, just behind us.

I can't find anything like this in my books, nor have I seen one like it before.

I need help. What do you think this is?

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