Exceptions are the following: -
a. Masculine are leó, leónis; ligó, ónis; sermó, -ónis; also cardó, harpagó margó, órdó, turbó; and concrete nouns in -ió: as, pugió, únió, papilió;[1][Many nouns in -ó (gen. -ónis) are masculine by signification: as, geró, carrier; restió, ropemaker; and family names (originally nicknames): as, Ciceró, Násó. §§ 236. c, 255.]
acínacés, ariés, celés, lebés, pariés, pés;
Nouns in -nis and -guis: as, ígnis, sanguis; also axis, caulis, collis, cucumis, énsis, fascis, follis, fústis, lapis, ménsis, orbis, piscis, postis, pulvis, vómis;
NOTE: Some nouns in -is and -ns which are masculine were originally adjectives or participles agreeing with a masculine noun: as, Aprílis (sc. ménsis), M., April; oriéns (sc. sól), M., the east; annális (sc. liber), M., the year-book.
b. Neuter are vás (vásis); crús, iús, pús, rús, tús.