Thallus appressed, loosely adnate, lobes 1-3 (-5) mm broad, short and rounded to more or less elongate; upper surface pale olive green, olive brown to reddish brown or blackening, smooth to wrinkled or pitted, lacking soredia or pseudocyphellae, sparsely or densely isidiate, the isidia more or less cylindrical, often branched, attenuate or knobby at the tips, easily broken off at the base to leave white spots, to 1.5 mm tall; lower surface dark brown to black, paler-toward the edges, rhizinate. Apothecia frequent, sessile, to 6 mm broad, flat or concave; margin entire when young, soon papillate to isidiate; hymenium 60-75 μ; spores ellipsoid to subglobose, 10-14 x 5.5-9.5 μ. Conidia 6-7.5 x 1 μ.

Reactions: cortex K— , HN03— ; medulla K— orK+ violet where orange pigmented, C+ red, P—.

Contents: lecanoric acid, and where the medulla is orange pigmented it contains rhodo-physin (skyrin).

This species grows on bark on or rocks. It is temperate circumpolar in distribution, in North America ranging south to North Carolina in the east and southern Calfiornia in the west, northward extending to the Arctic in the Mackenzie River valley.