AGATHYRNA, or AGATHYRNUM, AGATHYRRA, or AGATHYRNUM, in Ancient Geography, a town of Sicily; now St Marco; as old as the war of Troy, being built by Agathyrnus, son of Æolus, on an eminence. The gentilicious name is Agathyrneus; or, according to the Roman idiom, Agathyrnenfis.