Life habit: lichenized Thallus: thin to thick, placodioid, with radiate-plicate marginal lobes, areolate towards the center areoles: 0.3-1 mm wide, plane, often delimited by a dark, narrow prothallus surface: greenish yellow due to usnic acid in the cortex, color sometimes occluded by an outer epinecral layer, or a pruina of oxalate crystals (or both) Apothecia: frequent, cryptolecanorine becoming lecanorine, innate or more usually adnate, 0.2-0.9 mm in diam. disc: black or sometimes white-pruinose, plane to slightly convex; epihymenium: brown hymenium: hyaline, I+ blue; paraphyses: simple or weakly branched; hypothecium: hyaline asci: cylindrical, 8-spored ascospores: brown, 1-septate, 9-13 x 5-7 µm; becoming constricted centrally Pycnidia: immersed, occurring singly in the center of younger areolae, pyriform conidia: hyaline, simple, bacilliform, 5-6 x 1 µm long Spot tests: K+ yellow, C-, KC-, P+ red (chemotype I), K-, C+ red, P- (chemotype II), K-, C-, KC-, P- (chemotype III, VI), K+ yellow, C+ red, P+ red (chemotype IV), K+ yellow, C-, KC-, P+ orange (chemotype V), K+ yellow, C+ red, P+ orange (chemotype VII) Secondary metabolites: major compounds: usnic acid, fumarprotocetraric acid (chemotype I), usnic acid, gyrophoric acid (chemotype II), usnic acid (chemotype III), usnic acid, gyrophoric acid, fumarprotocetraric acid (chemotype IV), usnic acid, stictic acid (chemotype V), usnic acid, sphaerophorin (VI), usnic acid, gyrophoric acid, stictic acid (chemotype VII); main compounds often accompanied by norstictic acid, lecanoric acid and ovoic acid as minor compounds. Substrate and ecology: on steeply inclined surfaces of hard siliceous rocks World distribution: world-wide except in the tropics, Australasia and Antarctica Sonoran distribution: broadly distributed in the study area, Arizona, California, Baja California, Baja California Sur, Sonora and Chihuahua, from 400 to 2800 m; only four chemotypes (II, V, VI and VII) occur in the area. Notes: Dimelaena oreina is characterized primarily by the presence of usnic acid, giving the thallus a greenish yellow color. Brodo et al. (2001) refers specimens to Dimelaena suboreina with heavily pruinose apothecia and elongate lobes that are pruinose-scabrose. Due to the presence of intermediate forms and the lack of records in which the two morphotypes are found growing together, D. suboreina is treated as a synonym following Sheard (1974).