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Rhizocarpon simillimum (Anzi) Lettau
Family: Rhizocarpaceae
[Buellia simillima Anzi,  more]
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Life habit:  lichenized, not lichenicolous; Thallus: crustose, rimose-areolate, up to 5 cm in diam.; prothallus: black, well-developed; areoles: contiguous or scattered, rounded to angular, flat, up to 0.3 mm in diam.; surface: gray-brown to dark brown, dull; Apothecia: orbicular or angular to slightly flexuous, up to 0.8 mm in diam.; disc: black, +/- flat, epruinose; margin: narrow and persistent; exciple: brownish black; hypothecium dark brown; exciple and hypotheicum K+/- diffusing purplish red, without crystals or granules; hymenium: colorless or faintly brown; epihymenium: brownish black, K- or K+ faintly red, without crystals or granules; ascospores: soon dark black-gray-brown, 1-septate, 11-16 X 5-8 µm; Spot tests: medulla I+ blue-violet, K+ yellow or K-, C-; Secondary metabolites: none detected by TLC.

Substrate and ecology: on sunny, acid, +/- vertical rock faces; World distribution: Europe, North and South America; Sonoran distribution: Arizona and Chihuahua.

Notes:  In squash preparations in K, the hypothecium +/- exudes a reddish or magenta-purple color; other diagnostic features are the small, dark ascospores and the I+ blue medulla.  The anastomosing paraphysoids separate this species from Buellia.