Type. Rio Juruá superioris, am Juruá Miry, Amazonas, Brazil, Aug 1901, E. Ule 5670.
Krause does not describe the flowers in his original description, stating that “flores in specimine quod adest jam deflorati,” but in the type collection of this taxon at HBG the flowers are well preserved. They are herewith described: inflorescentia capitata 6 mm longa 7 mm lata 18-20-flora, axibus nullis; pedunculo tenui 5-7 mm longo, 0.8 mm lato puberulenti; floribus omnino sessilibus; calyce hypanthioque 1.5 mm longo glabro; hypanthio columnari 0.9 × 0.6 mm; calyce cupulari 0.6 mm longo 1.5 mm lato apice subtruncato vel paullo repando, lobis limboque erectis; calyce intus haud glandulifero; corolla tetramera subcylindrica 3.5 mm longa extus glabra, tubo 2.5 mm longo basi 1.2 mm longo fauce 1 mm lato intus parte basilari 1.3 mm glabro ceterum barbato, lobis quattuor late ovato-oblongis subobtusis apice paullo muticis inflexis 1.2 × 0.9 mm; staminibus manifeste exsertis; antheris lineari-oblongis rotundatis 1 mm longis; filamentis 1.5 mm longis; stylo 1.5 mm longo.
This taxon is closely related to P. moroidea Steyerm., from which it differs in the shorter, erect, non-squarrose calyx which is not ciliate on the margins, glabrous upper part of the stem, shorter and glabrous stipular teeth, and glabrous hypanthium. From P. pallidinervia Steyerm. it may be distinguished by the longer and puberulent peduncle, and truncate or subtruncate calyx border.
Standley identified the iso type specimen of P. nana at the Museu Goeldi Herbarium as P. cephalantha (M.-Arg.) Standi. The latter species, however, possesses completely sessile heads, whereas in P. nana they are shortly pedunculate. Other differences are that the petioles in P. cephalantha are much longer (7-11 mm) as compared with those of P. nana (2-5 mm), the calyx border in P. cephalantha is ciliolate, in P. nana glabrous, and in P. cephalantha there are 10 pairs of lateral nerves as contrasted to 6-7 pairs in P. nana.