A Grammatical Dictionary of Botanical Latin

-costus,-a,-um (in L. comp.); -costus,-a,-um (in L. comp.);

NOTE: there is no -costis,-e (adj.B).

Adjective compounds in -costus,-a,-um (adj.A) are mostly used in the feminine, seldom in the neuter, and mostly not in the masculine due to the fact that there is a generic name ‘Costus.’ Many epithets in ‘costus’ are nouns in apposition (this best seen when the genus name being modified by the epithet is in some other gender than masculine (usu. neuter)). There are few if any adjective compounds that end in -costus.

- ending in -costa: Acacia pubicosta; Ficus crassicosta; F. pubicosta; F. rubricosta; Frailea pygmaea var. planicosta; Goeppertia pallidicosta; Heliconia albocosta; Meteorium auricosta; Orthostichopsis auricosta.

Stephani used -costa in Metzgeria epithets: Metzgeria duricosta; Metzgeria nudicosta; Metzgeria sclerocosta; Metzgeria villosicosta

NOTE: these epithets are all compounds and probably do not refer to the genus name ‘Costa,-ae (s.f.I).

genus names:

Costa,-ae (s.f.I), Vell. Rutaceae; Costa armomatica.

Acosta,-ae (s.f.I) of various authors of Asteraceae (Compositae), Hypnaceae; Ericaceae, Polygalaceae.

- ending in -costum: Anthurium aspericostum; Anthurium glandulicostum; Phascum laticostum, Racosperma pubicostum

- ending in -costus:

Anthoceros pseudocostus; feminine genders, ‘costus’ as noun in apposition: Aucklandia costus; Saussurea costus; Theodorea costus

It appears that in most epithets ‘costus’ is a noun in apposition referring to the generic name ‘Costus;’ see costus.