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, 2-8 cm in diam., lobate lobes: subirregular, elongate, slightly imbricate, plane, separate, 5-8 mm wide; apices: rotund, eciliate upper surface: gray with some blackened areas, smooth, shiny, sometimes white pruinose, conspicuously white maculate soredia: granular, common, in linear to orbicular, laminal or marginal soralia; isidia and pustulae: absent medulla: white with continuous algal layer lower surface: black with brown, naked zone peripherally, centrally rhizinate; rhizines: scattered, simple Apothecia: not seen Pycnidia: not seen Spot tests: upper cortex atranorin K+ yellow, C-, KC-, P-; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with caperatic acid (major), protolichesterinic acid (minor). Substrate and ecology: on rocks in open habitats World distribution: neotropics and SW USA Sonoran distribution: lower montane of SE Arizona southwards through the thorn forests of Sonora and Sinaloa.
Type: Dominica. Saint Joseph: North of Coulibistri, ~ 30 m alt., Hale, M.E. 35649 (US – holotype, BM – isotype, UPS – isotype, fide Hale 1971).
Description.Thallus saxicolous; upper surface white to whitish gray, dull to ± shiny, epruinose, emaculate, occasionally cracked, but not forming a distinctly reticulate pattern; abundantly sorediate; soralia marginal, linear to ± subcapitate or almost coralloid (‘subfatiscent’); soredia ± granular, ± concolorous or slightly paler than the thallus surface, sometimes with a faint brownish tinge, pale inside; lobes moderate-sized, 5–7(–10) mm wide, rotund, axils incised, margins eciliate; lower surface 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 atranorin, praesorediosic and protopraesorediosic acid [P+ yellow, K+ yellow, KC–, C–, UV–].
Ecology and distribution. Africa, North, Central and South America (Hale 1971; Winnem 1975; Spielmann & Marcelli 2009; Egan et al. 2016). New for Galapagos; moderately common, from the coastal zone through the dry zone and transition zone into the humid zone, most abundant in the transition zone; exclusively saxicolous; mostly at sunny, wind- and rain-exposed sites, less frequently in ± shaded and sheltered habitats.
Notes.Parmotremamordenii is the only species known from the Galapagos that contains atranorin in its medulla. It is superficially very similar to P. praesorediosum, which, however, is mostly corticolous or lignicolous and only rarely grows on rock. The two species are best distinguished by their chemistry: both contain praesorediosic and protopraesorediosic acid within their medulla; however, Parmotremamordenii contains atranorin not only in its cortex but also in its medulla, which therefore reacts P+/K+ yellow. Medullary spot test reactions of P. praesorediosum are all negative. Morphologically, distinguishing the two species is difficult. Parmotremapraesorediosum has soralia that are more distinctly crescent-shaped, whereas they are linear to subcapitate in P. mordenii, or, if unusually well developed, even coralloid (Hale 1971 then uses the term ‘subfatiscent’).