PORTUNUS, or PORTUMNUS  , in Roman mythology, originally the god of gates and doors (Lat. porta), and as such identified with Janus and represented with a key in his hand . Gradually he came to be recognized as a separate deity, who protected the harbours (portus) and ensured a safe return to seafarers . (Cicero, Nat. dear. ii . 26; Virgil, Aen. v . 241) . With the introduction of the Greek gods, he became merged in Palaemon-Melicertes . He had a special priest (flamen portunalis) and temples on the Tiber near the Aemilian bridge and near Ostia, where a festival was celebrated in his honour on the 17th of August . Mommsen unhesitatingly identifies Portunus with the river-god Tiberinus, from the fact that the festival is also called Tiberinalia in the fasti of Philocalus; Marquardt regards him rather as the tutelary deity of warehouses . See J . Marquardt, Romische Staatsverwaltung (1885), iii . 327, note To .