Thallus: continuous to weakly fissured, with thin to very thin verrucae; margins: entire, unzoned upper surface: ash gray to greenish or yellowish gray, smooth to finely tuberculate, shiny or dull, epruinose or slightly pruinose; lacking soredia or isidia fertile verrucae: concolorous with thallus, ampliariate or pertusariate, few and well dispersed, c. 0.2-1.2 mm in diam.; ostioles: 1-5 per verruca, dark brown Apothecia: 1-3 (-5) per verruca; epithecium: dark brown to yellow-brown, K+ strongly violet; hypothecium: yellowish asci: cylindrical, 140-240 x 18-48 Ám, (6-) 8-spored ascospores: hyaline, oval to ellipsoid, 18-46 x 10-32 Ám; spore wall: 2-layered; outer spore wall: c. 1-3 Ám thick; inner spore wall: 1-4 Ám thick, smooth; apices: up to 8 Ám thick Pycnidia: not seen Spot tests: K- or + yellow, KC+ yellow, C-, P- or yellow to orange, UV- or UV+ weakly yellow Secondary metabolites: 4,5-dichloronorlichexanthone and stictic acid (both major) constictic acid and traces of unknowns (all minor). Substrate and ecology: on bark of Abies lasiocarpa World distribution: circumboreal in the Northern Hemisphere Sonoran distribution: mountainous areas in eastern Arizona at an elevation of 2600-3200 m altitude. Notes: This species is easily recognized by a thin, epiphloedal or hypophloedal, grayish thallus, porifom ascomata, a K+ violet reaction of the epithecium, 8-spored asci and the presence of stictic acid and 4,5-dichlorolichexanthone. This species is easily separated from most other Pertusaria species, except P. alpina Hepp ex Ahles or P. stenhammarii. The distinction from the latter is discussed below. Pertusaria alpina is distinguished from P. sommerfeltii in having a K- epithecium and larger ascospores. Megaspora verrucosa is superficially similar to P. sommerfeltii, which, however, is easily distinguished in lacking secondary metabolites, a K- epithecium, and a different ascus type.