§86. Gender in the Third Declension

Feminine are nouns in -ō, -ās, -ēs, -is, -ūs, -x, and in -s preceded by a consonant: as, legiō, cīvitās, nūbēs, avis, virtūs, arx, urbs. The nouns in -ō are mostly those in -dō and -gō, and abstract and collective nouns in -iō.

Exceptions are the following:—

a. Masculine are leō, leōnis; ligō,-ōnis; sermō, -ōnis; also cardō, harpagō margō, ōrdō, turbō; and concrete nouns in -iō: as, pugiō, ūniō, papiliō; ¹

acīnacēs, ariēs, celēs, lebēs, pariēs, pēs;

Nouns in -nis and -guis: as, īgnis, sanguis; also axis, caulis, collis, cucumis, ēnsis, fascis, follis, fūstis, lapis, mēnsis, orbis, piscis, postis, pulvis, vōmis;

mūs;

calix, fornix, grex, phoenīx, and nouns in -ex (gen. -icis) (§ 85);

dēns, fōns, mōns, pōns.

b. Neuter are vās ( vāsis); crūs, iūs, pūs, rūs, tūs.