Source: https://latin.stackexchange.com/questions/6657/adjectives-that-decline-as-consonant-stems-in-the-neuter-plural-nominative-accus
Timestamp: 2019-04-23 14:41:55+00:00

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From what I have read, most third-declension Latin adjectives other than comparatives take the i-stem endings -ī, ium and -ia in the ablative singular, genitive plural and neuter nominative/accusative plural, respectively.
Allen and Greenough §114-119 discuss the general patterns, and Allen and Greenough §121 discusses -e vs. -ī in the ablative singular, -ium vs. -um in the genitive plural, and a few other variations like -īs vs. -ēs in the masculine/feminine accusative plural and feminine nominative singular forms in -a, like clienta and hospita.
Adjectives that take -a in the neuter plural nominative/accusative appear to be even less common than adjectives that take -um or -e, though. It seems that many third-declension adjectives of one ending lack attested neuter plural forms at all (discussed to some extent in Allen and Greenough §122).
Which third-declension adjectives have attested plural forms in -a?
vetera. It is thought that vetus was originally a neuter substantive noun.
plūra (despite genitive plurium). It seems pluria was maybe a very uncommon variant form, but the typical form was plura. The derived word complura/compluria apparently exists in both forms.
ūbera. Of course, this exists as a plural substantive, but according to Allen and Greenough §119, the form ūbera is also used as a neuter plural adjective (although the ablative singular form of this adjective is usually ūberī). Wiktionary gives "ūberia", but that form doesn't show up anywhere in the PHI Latin Texts corpus, so I'm guessing Wiktionary is just showing an incorrect declension table for this word.
pūbera. Allen and Greenough §121d says that pūbes takes -a in the nominative and accusative plural, but I haven't found an actual example of pubera, so I suspect that this adjective might have been defective instead in Classical Latin. Wiktionary lists the neuter plural form as "puberia", which I would guess is an erroneous auto-generated form.
Guardia and Wierzejski (1875) include puber and memor in a list of adjectives that are defective in the neuter nominative/accusative plural (p. 133-134, Grammaire de la langue latine).
-corpora, -colora. Another old book ("Some improvements to the art of teaching...," Fifth Edition, by William Walker, 1693) mentions compound adjectives ending in -corpora or -colōra, like bicolōra, but Walker seems to say that these actually originated as o-stem forms (corresponding to masc. singular nominative -corporus, -colōrus), not as consonant-stem forms.
I found a few more mentioned in §3 of "Declension of the Latin present participle in connection with its syntactico-semantic use" (2018), by Hendrik Christiaan Walvoort.
Walvoort says that in a discussion of words that can take -i in the ablative singular but that have plurals forms in -a, Priscian mentions ludicra, which is supposed to go with the ablative form ludicri. However, it seems that Priscian is the main evidence for this as a third-declension adjective; dictionary entries record instead a first/second declension adjective ludicer or ludicrus.
Kühner and Holzweissig [...] consider the ending -a as a striking singularity instead of -ia in such phrases as silenta loca and pestilenta loca.
Not the answer you're looking for? Browse other questions tagged adiectivum morphologia declinatio declinatio-tertia or ask your own question.
Cur nullum genus adiectivo “vetus”?
Is -um (instead of -ōrum) a typical genitive plural ending outside of poetry?
Is there a neuter noun (word) that has different nominative and accusative form?

References: §114
 §121
 §122
 §119
 §121
 §3