Thallus: superficial (epiphloedal or epixylic), usually thick and well developed, squamulose, with large, convex verrucae, or thinner and then verrucose-granular, often forming extensive patches, olivaceous green to grayish green or pale brown, rarely +immersed photobiont: trebouxioid, rounded, clustered or solitary, not transversely septate, the cells 10-15 µm in diam. Apothecia: usually 0.5-1.2 mm long, 6-15 times as long as the width of the stalk stalk: rather stout, 0.06-0.1 mm in diam., dark brown to pale brown, epruinose or with a faint yellowish green pruina over the uppermost part; outermost part: hyaline, 716 µm thick, consisting of strongly gelatinized, periclinally arranged hyphae capitulum: broadly obconical to lenticular, 0.20-0.36 mm in diam. exciple: well developed, consisting of largely periclinally arranged, slightly branched, brown hyphae, with a +dense yellowish green pruina on the lower side; hypothecium: broadly obconical with flat or slightly concave upper surface asci: cylindrical to narrowly clavate, 17-24 x 3-4.5 µm, with a long stalk, produced singly from branched ascogenous hyphae with hooks; with uni- to biseriate spores ascospores: spherical, 5.5-7 µm in diam. when mature, irregular reticulate fissures in the mature spores give rise to an ornamentation of irregular polygonal areas Spot tests: thallus K-, C-, KC-, P- Secondary metabolites: vulpinic acid in the pruina on the lower side of the capitulum. Substrate and ecology: on wood of coniferous (Picea, Larix, Pinus) and deciduous trees or sometimes bark of various other trees, including Quercus World distribution: widely distributed in the boreal zones of Eurasia and North America Sonoran distribution: California. Notes: Chaenotheca phaeocephala is characterized by its well developed exciple, that is greenish yellow pruinose on the lower side, by its rather short and robust apothecia, by its usually well developed verrucose-squamulose thallus, and by its rather large, spherical spores, and the association with a trebouxioid photobiont. It has a wide range of morphological variation in the shape of the apothecia and the development of the thallus. Specimens from bark have a very thin and scurfy thallus and small apothecia; specimens on wood usually have a thick and almost squamulose thallus and larger apothecia. Specimens from high elevations have a strikingly well developed thallus with convex verrucae, a rather thick cortex and slender apothecia. Chaenotheca phaeocephala is closely related to C. subroscida (see notes under that species).