[*] 48. Nouns ending in -us (-os), -er, -ir, are Masculine; those ending in -um (-on) are Neuter. Exceptions: Names of countries and towns in -us (-os） are Feminine: as, Aegyptus , Corinthus . Also many names of plants and gems, with the following: alvus, belly; carbasus, linen (pl. carbasa, sails, N.); colus, distaff; humus, ground: vannus, winnowing-shovel. Many Greek nouns retain their original gender: as, arctus (F.), the Polar Bear; methodus (F.), method. [*] a. The following in -us are Neuter; their accusative (as with all neuters) is the same as the nominative: pelagus, sea; vīrus, poison; vulgus (rarely M.), the crowd. They are not found in the plural, except pelagus , which has a rare nominative and accusative plural pelagē .
[*] Note.--The nominative plural neuter cētē, sea monsters, occurs; the nominative singular cētus occurs in Vitruvius.