Allen and Greenough's New Latin Grammar, section 31

Heavy Construction

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Names of Male beings, and of Rivers, Winds, Months, and Mountains, are masculine: -

pater, father; Iúlius, Julius; Tiberis, the Tiber; auster, south wind; Iánuárius, January; Apennínus, the Apennines.

NOTE: Names of Months are properly adjectives, the masculine noun ménsis, month, being understood: as, Iánuárius, January.

a. A few names of Rivers ending in -a (as, Allia), with the Greek names Léthé and Styx, are feminine; others are variable or uncertain.

b. Some names of Mountains are feminine or neuter, taking the gender of their termination: as, Alpes (F.), the Alps; Soracte (N.).