Answer:
RULE 1: I-stem third-declension nouns: (1) are "parisyllabic"; (2) have a monosyllabic nominative singular ending in -s/x and two consonants at the end of the base; (3) or, are neuters ending in -e, -al or -ar.
You have to look closely, but the "-e-" of the stem is short. This is the characteristic vowel of the third conjugation: short "-e-".
117. The remaining adjectives of the 3rd declension are Consonant stems; but most of them, except Comparatives, have the following forms of i-stems: -ī in the ablative singular (but often -e). ... -īs (as well as -ēs) in the accusative plural masculine and feminine.
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