Surplus to requirement

I found this pretty bowl abandoned on the shore. It was a few inches wide. The former owner had no further use for it and it washed up on the beach.
Its owner was a spider crab that shrugged off this shell and grew a larger one.
This is the underside of the carapace, the part that would have contained the crab's soft body. It looks like porcelain,  tinged a delicate blue with  holes and dimples perhaps corresponding to the tubercles on the spiny exterior surface.
 I couldn't get over how beautiful it was.

Extra shows the whole thing.
(And see how the barnacles have all had babies!)