Allen and Greenough's New Latin Grammar, section 73

Heavy Construction

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Summary of i-Stems.

73.

The i-declension was confused even to the Romans themselves, nor was it stable at all periods of the language, early Latin having i-forms which afterwards disappeared. There was a tendency in nouns to lose the i-forms, in adjectives to gain them. The nominative plural (-ís)[1][An old, though not the original, ending (see p. 32, footnote 2).] was most thoroughly lost, next the accusative singular (-im), next the ablative (-í); while the genitive and accusative plural (-ium, -ís) were retained in almost all.