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DescriptionThallus: irregular, rarely orbicular, small, up to 1 cm diam., ± loosely adnate, rarely confluent with other thalli to form bigger patches; lobes: ± ascending, narrow, up to 1 mm, but usually around 0.5 mm, sometimes widening at tips; margins: rarely having gray to black cilia; upper surface: gray to dark gray or brownish, without pruina, with white to black hairs, sorediate; soredia: in terminal to marginal soralia, which when well developed widen the lobe tips, occasionally covering the whole lower surface of the lobe; upper cortex: paraplectenchymatous; medulla: white; lower cortex: prosoplectenchymatous; lower surface: white to brownish, with few white to black rhizines; Apothecia: rare; ascospores: brown, 1-septate, Pachysporaria-type (15-) 16.5-21 (-22.5) x (7.5-) 8-10.5 (-12) µm; Pycnidia: not common, immersed, appearing as black warts; conidia: subcylindrical, 4-6 x 1 µm; Spot tests: cortex K+ yellow, C-, KC-, P+ yellow; medulla K-, C-, KC-, P-; Secondary metabolite: upper cortex with atranorin.; Substrate and ecology: growing both on rocks and trees in open situations; World and Sonoran distribution: presently known from Baja California and southern California.; Notes: Physcia tenellula is closely related to P. tenella (Scop.) DC and has previously been identified as the latter species. They differ, however, in size and presence of hairs on the upper surface. The restricted distribution to the islands outside the coast also gives evidence for P. tenellula being a separate taxon.