a derivative form from summus, the highest, an ancient Roman or Etruscan divinity, who was equal or even of higher rank than Jupiter; in fact, it would seem that as Jupiter was the god of heaven in the bright day, so Sum- inanus was the god of the nocturnal heaven, and lightnings plying in the night were regarded as the work of Summanus.

Varro (De Ling. Lot. v. 74) describes the god as of Sabine origin; but the ancients themselves on this as on many other points connected with their earliest religion, were in great uncertainty both in regard to the nature and the origin of Summanus and some connecting the name with sub and manes regarded him as a deity of the lower world, an opinion which is totally at variance with the attributes given him by most writers, and there is ample reason for regarding him as the Jupiter of night. He had a temple at Rome near the Circus Maximus ( Ovid Fasti vi.). There was a representation of Summanus in the pediment of the Capitoline temple.

From the Dictionary of Greek and Roman Biography and Mythology.