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DescriptionLife habit: lichenized, not lichenicolous; Thallus: crustose, 2-7 cm in diam.; prothallus: black, rather thick, with a radiating or diffuse edge; areoles: peltate, 0.6-1.3 mm in diam., 0.25-0.35 mm thick; margins: black or gray, thick, prominent, incurved, generally wavy sinuate, incised, sub-pulverulent; surface: brown or reddish brown, concave or subplane, shiny or dull; cortex: upper cells brown or brownish violet, with hyphae 4-7 µm wide, 25-40 µm thick, with an epinecral layer 20-40 µm thick that frequently sloughs off; medulla: white, cells almost isodiametric, 5-6 µm wide; lower side dark, often easily visible, narrowly fixed to substrate; Apothecia: dispersed, adnate or sessile between the areoles, (0.3-)0.6-1 mm in diam., 0.2-0.35 mm thick; disc: black, finally rugulose and convex, epruinose; margin: black, persistent or disappearing; exciple: black or brownish violet peripherally, lighter internally, 60-70 µm thick in lateral part, in outer zone, K+ more distinctly violet; epihymenium: brownish violet, K- or K+ distinctly violet; hymenium: hyaline or pale violet, 100-135 µm tall; paraphyses: loosely conglutinate, branched, septate, tips fused into a pseudoparenchymatous tissue, 2-3 µm broad below, 3-5 µm apically; asci: saccate or inflated, 80-110 x 22-30 µm, (1-)2-spored; ascospores: hyaline then green or violet, finally dark brown, 3-septate to usually muriform, oblong-ellipsoid, (32-)40-50(-68) x 18-32 µm; wall: c. 2 µm thick; halo 3-4 µm wide, in K 4-6 µm wide; Spot tests: cortex K-, C, KC-, P-; medulla I-, K+ yellow or K-, C-, KC-, P-; Secondary metabolites: not determined.; Substrate and ecology: on siliceous rocks, often on seepage surfaces but also on drier, often manured, surfaces; World distribution: western North America, Greenland and Norway; Sonoran distribution: southern California at 500-2300 m and Baja California.; Notes: Rhizocarpon bolanderi is very common at least in the Sierra Nevadas. Superficially R. bolanderi is very similar to some brown Lecidea species, but its areoles are usually smaller, more concave, darker, and shinier. A chemotype with yellow medulla, which occurs in northern California, has been described as R. bolanderi var. sulphurosum (Tuck.) Zahlbr. It may represent a distinct taxon.