Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2002. Lichen Flora of the Greater Sonoran Desert Region. Vol 1.
Thallus: foliose, adnate, 3-10 cm in diam., lobate lobes: subirregular, elongate, slightly imbricate, plane, separate, 4-10 mm wide; apices: rotund, often suberect, eciliate upper surface: gray, smooth, dull, emaculate to faintly white maculate, becoming cracked with age soredia: granular, common, in linear to crescent-shaped, marginal or submarginal soralia; isidia and pustulae: absent medulla: white with continuous algal layer lower surface: black with brown to mottled ivory naked zone peripherally, centrally rhizinate; rhizines: scattered, simple Apothecia: rare, substipitate, up to 10 mm in diam.; margin: sorediate; disc: brown, imperforate ascospores: ellipsoid, 15-21 x 7-10 µm Pycnidia: rare, punctiform conidia: sublageniform, 7-8 x 1 µm Spot tests: upper cortex K+ yellow, C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with protopraesorediosic and praesorediosic acids (major), unknown fatty acids (minor). Substrate and ecology: on rocks and trees in open habitats World distribution: pantropical and pantemperate Sonoran distribution: thorn forests of Sonora, Sinaloa and southern Baja California Sur. Notes: Originally the species was reported to have caperatic acid (Hale 1965), but subsequent investigation has shown the fatty acids to be unique (Elix 1994k).
Type: Singapur [State or Province, and exact locality unknown]. 30-Nov-1879, Almquist, E. s.n. (H-NYL 35547 – holotype, S – isotype, fide Fleig 1997).
Description.Thallus corticolous, lignicolous, rarely saxicolous; uppersurface white to whitish gray, dull to ± shiny, epruinose, emaculate, occasionally cracked, but not forming a distinctly reticulate pattern; abundantly sorediate; soralia marginal, linear to labriform (± crescent-shaped), or subcapitate; soredia ± granular, ± concolorous or slightly paler than the thallus, their surface often discolored by a dark gray tinge, pale inside; lobes moderate-sized, 5–7(–10) mm wide, rotund, axils incised, margins eciliate; lowersurface with an erhizinate, ~ 1–2(–2.5) mm wide, deep brown margin, moderately to densely rhizinate and blackened towards the center; rhizines short, stout, black, simple to sparsely branched; medulla white. Apothecia and pycnidia not observed among the Galapagos specimens.
Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with ± caperatic, praesorediosic and protopraesorediosic acid [P–, K–, KC–, C–, UV–].
Ecology and distribution. Oceania, Africa, Asia, North, Central and South America (Hale 1965; Swinscow & Krog 1988; Marcelli 1992; Elix 1994; Louwhoff & Elix 1999; Divakar & Upreti 2005; Jungbluth 2006; Spielmann & Marcelli 2009; Marcelli & Benatti 2010; Benatti & Marcelli 2010; Egan et al. 2016). First reported from Galapagos by Weber (1986), subsequently by Elix & McCarthy (1998) and online (Bungartz et al. 2016). A common species in the Galapagos from the coastal zone all the way into the humid zone, most frequently encountered in the dry zone; typically on bark, rarely on rock, most frequently on Bursera graveolens, occasionally on other, mostly native or endemic trees and shrubs.
Notes. Differences from the morphologically similar P. mordenii have been discussed under that species.