foliose, closely adnate to the substratum, commonly dying in the center and forming concentric rings, the lobes irregularly branching, the tips pointed, flat to slightly convex, 2 mm broad; upper side greenish yellow, partly whitish green, darkening toward the center; dull, sometimes chinky, lacking soredia or isidia; underside whitish or pale brownish with brown to blackish rhizines; medulla
adnate, to 5 mm broad, the margin entire to crenulate or inflexed, concolorous with the thallus; disk reddish or dark brown, dull; epithecium brownish; hypothecium
and hymenium hyaline; spores ellipsoid, 8-14x4.5-6 μ.
Reactions: hymenium 1+ blue; cortex K+ yellow; medulla I— , K— , KC+ red, P— .
Contents: usnic acid, alectoronic acid, atranorin.
This species grows on rocks, preferably acid, usually in strong light but in more shady spots becoming softer in texture. It is circumpolar, arctic-alpine, ranging south in North America into the Appalachian Mountains and near the northern shores of the Great Lakes.