AGATHYRNA, or AGATHYRNUM, AGATHYRISA, or AGATHYRNUM, (anc. geog.), a town of Sicily; now S. Marco; as old as the war of Troy, being built by Agathyrnus, son of Æolus, on an eminence. The gentilicious name is Agathyrnacus; or, according to the Roman idiom, Agathyrnenfis.