The Adventure of the Lion's Mane

Holmes is enjoying his retirement in Sussex when one day at the beach, he meets his friend Harold Stackhurst, the headmaster of a nearby preparatory school called The Gables. No sooner have they met than Stackhurst's science teacher, Fitzroy McPherson, staggers up to them, clearly in agony and wearing only an overcoat and trousers. He collapses, manages to say something about a "lion's mane", and then succumbs. He is observed to have red welts all over his back, possibly administered by a flexible weapon of some kind, for the marks curve over his shoulder and round his ribs.