[*] 86. 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.
[*] Note.--Some nouns in -is and -ns which are masculine were originally adjectives or participles agreeing with a masculine noun: as, Aprīlis (sc. mēnsis ), M., April; oriēns (sc. sōl ), M., the east; annālis (sc. liber ), M., the year-book.
[*] b. Neuter are vās ( vāsis ); crūs , iūs , pūs, rūs , tūs.