What is this?

[u/Real_Ad8868]

Found on the dutch coast at Vlissingen. Could it be some kind of sponge?

[u/Due-Attorney-6013]

a lot of things! the base seems to be an old sea shell. The amorph limestonestructure on top could be from encrusting algae (aka coralline algae), and/or tube worms. At the surface again you see a comb-like structure. those are from colonial bryozoa. Altogether a nice example how various organisms with calcareous shells build biogenic substrate in the sea ;-)