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; vírus, poison; vulgus (rarely M.), the crowd. They are not found in the plural, except pelagus, which has a rare nominative and accusative plural pelagé.
NOTE: The nominative plural neuter cété, sea monsters, occurs; the nominative singular cétus occurs in Vitruvius.