Thallus: prostrate and decumbent, in extreme forms becoming almost subfoliose or even virtually crustose, generally forming ± circular adpressed thalli 2-6 cm in diam., closely attached, adnate at the tips branching: irregularly isotomic-dichotomous with numerous short lateral branchlets giving thallus a "ciliate" appearance; internodal distance: short, usually c. 0.2-0.5 (-1.0) mm, giving the thallus a rather dense appearance branches: becoming ± flattened, especially towards the base, unevenly thickened, up to c. 1 mm broad, tapering irregularly to 0.2-0.5 mm on main stems and 0.1 mm at tips, where the branches are very uneven and ± terete upper surface: dark brown to black, shiny (to dull), sometimes pale brown at the base; true lateral spinules: absent, but simple or branched isidioid adventitious branches not uncommon in one morphotype; soralia and pseudocyphellae: absent lower surface: dark brown Apothecia: often present, lateral; disc: 1-3 mm broad, dark red-brown to black, plane; thalline exciple: persistent, smooth or verrucose asci: clavate, 8-spored ascospores: broadly ellipsoid to subglobose, 6.7-9.0 x 7.7-8.0 µm Pycnidia: very abundant, embedded in thallus, ostioles: broad conidia: bifusiform, 4.7-8 x 0.9-1.1 µm Spot tests: K-, C-, KC-, P-, UV- Secondary metabolites: none detected. Substrate and ecology: on acidic or occasionally base-rich rock faces, boulders, and windswept gravels. In exposed sites, in drier inland areas, in temperate areas mostly in alpine sites World distribution: circumpolar in both the Northern and Southern Hemispheres Sonoran distribution: moderately frequent at high alpine (3000-3650 m) in north-central to eastern Arizona, occasionally lower, also known from three montane localities in southern California. Notes: It is highly variable and sometimes rather similar to P. pubescens, but usually separated from that species by the partly broader, more flattened and knobby branches (especially towards the base) and the short internodes. Specimens (including some from Arizona) with especially broadened lobes have occasionally been confused with Cornicularia normoerica, which has an ± erect and sparingly branched thallus and terminal apothecia with dentate or fibrillate margins, or with species of Melanelia or Neofuscelia, both of which have more flattened lobes usually with rhizines on the underside.