One More Mystery Sea Creature

This time Little-Shirley-Sharp-Eyes was looking in the other direction so it was I who spotted this Thing, about 2.5cms across, lying on a stone on the Coromandel peninsula, waiting for the tide to take it back to sea.

At first I thought that one end was missing - damaged or eaten, perhaps, but when viewed in big (very big) there does appear to be a definite structure.

I have no idea what it is so all suggestions welcome.