I found this 4cm wide "knitted mushroom cap" at a beach near Kenosha in SE Wisconsin. I thought at first to have found a tabulate coral, but looking closer, I can't see any corallites at all and oddly, the top, instead of flaring out to a solid "table", curves back into itself, with most of the center missing altogether.
All of this made me think that maybe this is a sponge? If not, what could it be?
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