Neptune is one of only four Roman gods to whom it was considered appropriate to sacrifice a bull. This basilica supplanted the older temple, which had replaced an ancient altar. The Basilica Neptuni was later built on the Campus Martius, and was dedicated by Agrippa in honor of the naval victory of Actium. Within the temple was a sculpture of a marine group by Scopas Minor. 40 BC, an event depicted on a coin struck by the consul. The temple was restored out by Gnaeus Domitius Ahenobarbus c.
It stood near the Circus Flaminius, the Roman racetrack in the southern part of the Campus Martius, and dates back to at least 206 BC. There is an added context of agricultural fertility in the festival, since Neptune received the sacrifice of a bull.
It was a time of merrymaking, when men and women could mix without the usual Roman societal constraints. Neptunalia was spent under branch huts in a woods between the Tiber and the Via Salaria, with participants drinking spring water and wine to escape the heat. These culminated in the Furrinalia, sacred to Furrina (the goddess of springs and wells). Neptunalia followed, devoted to conservation and the draining of superficial waters. The Lucaria was devoted to clearing overgrown bushes and uprooting and burning excess vegetation. It has been speculated that the three festivals fall in a logical order. All three festivals were connected to water during the period of summer heat ( canicula) and drought, when freshwater sources were lowest. The most ancient Roman calendar set the feriae of Neptunus on July 23, between the Lucaria festival of the grove and the Furrinalia festival of July 25. The date of the festival and the construction of tree-branch shelters suggest that Neptune was a god of water sources in times of drought and heat. Neptunalia, the Roman festival of Neptune, was held at the height of summer (typically on July 23). Neptune was considered the legendary progenitor god of the Falisci (who called themselves Neptunia proles), joining Mars, Janus, Saturn, and Jupiter as the deific father of a Latin tribe. For a time, Neptune was paired in his dominion of the sea with Salacia, the goddess of saltwater. By the first century BC, he had supplanted Portunus as the god of naval victories Sextus Pompeius called himself the "son of Neptune". Neptune has been associated with a number of other Roman deities. This is in contrast to Poseidon, who was primarily a god of the sea. Servius explicitly names Neptune as the god of rivers, springs, and waters he may parallel the Irish god Nechtan, master of rivers and wells. It has been speculated that Neptune has been conflated with a Proto-Indo-European freshwater deity since the Indo-Europeans lived inland and had little direct knowledge of the sea, the Romans may have reused the theology of a previous freshwater god in their worship of Neptune. The lectisternium of 399 BC indicated that the Greek figures of Poseidon, Artemis, and Heracles had been introduced and worshipped in Rome as Neptune, Diana, and Hercules. The theology of Neptune is limited by his close identification with the Greek god Poseidon, one of many members of the Greek pantheon whose theology was later tied to a Roman deity. Detail of a large Roman mosaic from Cirta, Roman Africa (ca. Triumph of Poseidon and Amphitrite, showing the couple in procession.