53. Nouns of the 3rd Declension end in a, e, ī, ō, y, c, l, n, r, s, t, x.
54. Stems of the 3rd Declension are classed as follows.
I. Consonant Stems
a. Mute stems
b. Liquid and Nasal stems
a. Pure i-stems
b. Mixed i-stems
55. The Nominative is always derived from the stem. The variety in form in the Nominative is due to simple modifications of the stem, of which the most important are—
1. Combination of final consonants, as of c (or g) and s to form x.
dux, ducis, stem duc-
rēx, rēgis, stem rēg-
2. Omission of a final consonant, as of a final nasal.
leō, leōnis, stem leōn-
ōrātiō, ōrātiōnis, stem ōrātiōn-
3. Omission of a final vowel, as of final i.
calcar, calcāris, stem calcāri-
4. Change of vowel in the final syllable, as of a to e.
prīnceps (for -caps), prīncipis, stem prīncip- (for -cap-)