Thallus: appressed, up to 6 (-8) cm broad; texture: cartilaginous; branching: isotomic dichotomous, budding occasional lobes: contiguous to imbricate or ± separate, 0.5-2.5 (-4) mm broad; black border: not visible from above; profile: even to irregular; width/height ratio: 1-4; tips and axils: entire or torn upper surface: white, gray to greenish gray, dark mottles none or rare, occasionally rugose soredia: on the inside of the burst lobe tips, appearing as if in labriform soralia; isidia absent, lobules rare medulla: hollow, ceiling of cavity white or dark, floor of cavity dark lower surface: black, rarely perforate Apothecia: rare, substipitate to stipitate, up to 2 (-4) mm in diam; stipe: funnel-shaped, hollow; disc: brown ascospores: ellipsoid, 7-8 x 4.5-5.5 µm Pycnidia: occasional conidia: rod shaped, 5.5-5.8 x 0.5-0.6 µm Spot tests: cortex K+ yellow, C-, KC-, P+ pale yellow, UV-; medulla K-, C-, KC+ orange-red, P+ orange-red. Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with physodic acid (major), 2'-O-methylphysodic acid (minor or accessory), 3-hydroxyphysodic acid (major), physodalic acid (major), and protocetraric acid (minor). Substrate and ecology: on bark or wood including conifers and hardwoods, rarely on rock, moss, or alpine sod World distribution: circumpolar arctic, boreal, and montane; northern Asia, North America, and Europe; throughout northern North America, south in California to about Santa Cruz and S throughout the Appalachian and Rocky Mountains Sonoran distribution: southern Rocky Mountains. Notes: Although this generalist species is widespread and common in the northern hemisphere, in the southwestern U.S. and Mexico it is apparently restricted to the higher mountains in a continental climate. It is remarkably consistent in chemistry considering its wide geographic range and the variability in chemistry of so many Hypogymnia species. Occasional small esorediate individuals can be recognized by the P+ medulla and imperforate lobe tips.