Type: Brazil. Glaziou, A.F.M. 1833 (M – holotype, H – isotype, fide Hale 1965).
Description.Thallus saxicolous; uppersurface pale greenish yellow, dull to ± shiny, emaculate, but especially the older parts frequently with conspicuous, ± reticulate cracks; abundantly isidiate; isidia both marginal and laminal, cylindrical and simple to abundantly branched, becoming coralloid, brown to blackened at their tip and frequently ciliate; lobes moderate-sized, 2–8 mm wide, rotund, axils rounded, forming ± indistinct loops, margins sparsely to densely ciliate; cilia short and slender, 0.5–2.5 mm long, black, mostly simple, very rarely branched; lowersurface with a deep brown, erhizinate, ~ 2–5 mm wide margin, soon blackened and densely rhizinate towards the center; rhizines long, slender, black, mostly simple, rarely sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Chemistry. Cortex with usnic acid [P–, K–, KC± dirty yellowish brown, C–, UV–]; medulla with salazinic and gyrophoric acid [P+ strongly yellow, K+ yellow, quickly turning blood-red, C+/KC+ pale pink (C and KC reactions are evanescent and must be examined under continuous observation); UV–].
Ecology and distribution. Africa (Müller Argoviensis 1880), Central America (Hue 1899; Hale 1965; Tenorio et al. 2002), South America (Hale 1965). In South America most reports are from Brazil (Krempelhuber 1869, 1876; Nylander 1885; Zahlbruckner 1909; Hale 1965; Fleig & Riquelme 1991; Fleig 1997; Ribeiro 1998; Eliasaro 2001; Benatti & Marcelli 2009; Donha 2005; Spielmann 2009). New for Galapagos and Ecuador; moderately common; most specimens found in the transition zone and humid zone of several islands, generally growing on sunny, wind- and rain-exposed rock surfaces (boulders, outcrops and cliffs).
Notes. The morphology of saxicolous P. flavescens is extremely similar to its corticolous counterpart, P. nyasense. Apart from inhabiting different substrates, the two species also differ by their chemistry. Both share gyrophoric acid within their medulla, but P. flavescens additionally contains salazinic acid.