Allen and Greenough's New Latin Grammar for Schools and Colleges
J. B. Greenough, G. L. Kittredge, A. A. Howard, Benj. L. D'Ooge, Ed.

ācer, ācris, ācre, keen, STEM ācri-

[*] a. Like ācer are declined the following stems in ri-:—

1. alacer, campester, celeber , equester, palūster, pedester, puter , salūber , silvester, terrester, volucer. So also names of months in -ber: as, Octōber (cf. § 66).

[*] Note 1.--This formation is comparatively late, and hence, in the poets and in early Latin, either the masculine or the feminine form of these adjectives was sometimes used for both genders: as, coetus alacris (Enn.). In others, as faenebris , fūnebris , illūstris , lūgubris , mediocris , muliebris , there is no separate masculine form at all, and these are declined like levis (§ 116).

[*] Note 2.-- Celer , celeris , celere, swift, has the genitive plural celerum , used only as a noun, denoting a military rank. The proper name Celer has the ablative in -e.