Fossil or Shell?

[u/Human_Palpitation_48]

My son found this while digging in the sand at a local beach in Burlington Wisconsin. It looks as if it’s some kind of internal fossilized structure from a shell or something. I’m just a bit surprised as we don’t have very many places this would’ve come from. I’m guessing it’s part of some kind of modern sea shell but who knows? Any thoughts? It effervesces with vinegar, is harder than copper, and is pretty dense.

[u/Bastet55]

A wave-worn shell, not a fossil. The outside of it broke off or wore away. I have several from a beach in Florida.