[45]. Exceptions to the Rule for Feminines.

1. Nouns in -ās.

a. Masculine: vās, bondsman.

b. Neuter: vās, vessel.

2. Nouns in -ēs.

a. Masculine: ariēs, ram; pariēs, wall; pēs, foot.

3. Nouns in -is.

a. Masculine: all nouns in -nis and -guis; as, amnis, river; īgnis, fire; pānis, bread; sanguis, blood; unguis, nail.

Also—

axis, axle.
collis, hill.
fascis, bundle.
lapis, stone.
mēnsis, month.
piscis, fish.
postis, post.
pulvis, dust.
orbis, circle.
sentis, brier.

4. Nouns in -x.

a. Masculine: apex, peak; cōdex, tree-trunk; grex, flock; imbrex, tile; pollex, thumb; vertex, summit; calix, cup.