Thallus: umbilicate, monophyllus, up to 15 cm in diam., with abundant or scattered pustules; margin: undulate to laciniate; margin and pustules: occasionally developing adventitious lobes upper surface: dark brown to blackish brown, dull or shiny, sometimes slightly areolate towards the center of the thallus upper cortex: texture formed mainly from anticlinal hyphae, pallisade plectenchyma, with a brown walled layer of cells on the upper part, with or without amorphous epicortex, 15-40 µm thick medulla: composed of two layers; upper part: 15-30 µm thick, arachnoid, with crystalline hyphae and leptodermous; lower part: prosoplectenchymatous, with pachydermatous hyphae of variable thickness, decreasing when deeping into pustules, sometimes disappearing towards the upper layer, up to 65 µm thick in the intermediate, non-pustulate thallus area algal layer: continuous, 25-40 µm thick lower cortex: scleroplectenchymatous, comprising two zones, a lower, up to 120 µm thick, strongly necrotic zone, and an upper, translucent zone, up to 150 µm thick lower surface: uniformly black, strongly verrucose-areolate Apothecia: common, up to 2 mm in diam., sessile to subpedicellate; disc: flat, black; margin: smooth to slightly crenate asci: clavate, 1-spored ascospores: initially pale brown, becoming dark brown, muriform, 33-60 x 15-30 µm Spot tests: upper cortex and medulla K-, C+ red, KC+ red, P- Secondary metabolites: gyrophoric acid. Substrate and ecology: growing on sandstones, granites, gneisses and other acid rocks, on xeric surfaces World distribution: Asia and North America, reaching into the Arctic Sonoran distribution: much less common than L. papulosa, Arizona, Chihuahua, and Baja California, over 2000 m in upper montane forests. Notes: The characteristic feature of Lasallia pensylvanica is its necrotic lower cortex. Superficially it is somewhat similar to L. rossica Dombr. from Eurasia (Sancho and Crespo 1989), that has laminal isidia and paraplectenchymatic lower cortex and lacks the necrotic layer.