Thought it looked slightly acorn-like in the thumbnail,
with the leaf where the cup would be....

Taken on the Beech Avenue at Kingston Lacy (its natural habitat),
this is a 'coprinus picaceus', or 'magpie inkcap'; in French, 'coprin noir et blanc'.
It's probably poisonous, but has supposedly been eaten by some with no ill-effect. Um....

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