1911 Encyclopædia Britannica/Portunus
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. deor. 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, Römische Staatsverwaltung (1885), iii. 327, note 10.