Cephaelis trichophoroides (M.-Arg.) Standl., Field Mus. Publ. Bot. 8: 183. 1930.
Petagomoa nigricans Brem., Rec. Trav. Bot. Neerl. 31: 295. 1934.
Like P. sciaphila S. Moore the flowering heads possess 7-8 main bracts, of which the outermost 4 surround the head and are united at the base for 1-2 mm. The four outermost bracts surround four interior ones, which may be narrower or more reduced in size. The 2-8 flowers within a head are not subtended by either bracts or bracteoles. Also, like P. sciaphila, the stems are glabrous below and over most of the internodes, but pilose-villous or hirsute at the nodes. From P. sciaphila it is differentiated in part by the glabrous peduncles.
It is not possible to separate Petagomoa nigricans Brem. from Psychotria trichophoroides. Both show the same type of pubescence and length of stipules, setose calyx lobes and ciliate bracts, linear-setaceous calyx lobes, similar length of petiole, similar shape and size of leaf blades with strongly spreading stiff hairs on the lower side of the midrib.
Although Bremekamp did not see fruits or seeds of Petagomoa nigricans (Rec. Trav. Bot. Néerl. 31: 295. 1934), nevertheless he placed the species in his genus Petagomoa. However, the individual flowers of this taxon do not possess their own bracts and bracteoles, a character basic to this genus, according to Bremekamp’s concept and separation of the genus in his key from other related genera (Rubiaceae of Suriname, in Pulle, FI. Surin. 4(1): 123. 1934). Bremekamp also describes the involucral bracts of Petagomoa as free to the base or nearly so, but in Petagomoa nigricans they are partially united for 2 mm at the base to form a short tube. Although Bremekamp describes the involucral bracts as several, there are, as indicated above, four outermost ones and four inner ones of essentially the same length or quite reduced in size. The characterization of Petagomoa as comprising “pseudo-dichotomously branched shrublets” (Rec. Trav. Bot. Néerl. 31: 294. 1934) and “suffrutices erecti” (p 295) certainly does not hold true for such species as P. nigricans (= P. trichophoroides), which has the habit, according to the collectors’ notes on the labels of “half-creeping herb,” “planta rastrera,” or “herbe ±enracinée aux noeuds.”