In the protologue of P. campylotropa, Hill cites P. umbilicata Ruiz & Pav. as its synonym without any restriction, which makes the first superfluous for the second. Although nomenclaturaly, P. campylotropa is a (superfluous, hence illegitimate) synonym of P. umbilicata, the latter species has a strictly Peruvian coastal distribution and none of the cited collections agrees with that species.
All cited specimens, except the South American von Humboldt s.n. (Willd. 764), are from Mexico and belong to several other species. Parry & Palmer 802 belongs to P. gracillima Watson and that is likely the reason for which P. campylotropa has been suggested as a synonym for P. gracillima (Nyffeler & Rowley, 2002: 365). P. gracillima, however, is a rather rare species restricted to the shady and wet environment of narrow canyons and cave entrances and not agreeing with the other cited collections. Trelease considered Bourgeau 418 and 631 as P. painteri Trel. (G specimen annotation) and Pringle 4124 represents P. bracteata A.W. Hill. The latter is the correct name for most of the herbarium specimens identified as P. campylotropa, illustrating that most later botanists understood Hill’s concept that way (Mathieu, 2009, [pers. comm.]).