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DescriptionThallus: areolate-warty, rather thin, often bordered by a blackened base, sometimes obscurely lobed; areoles: when still flat 0.5-0.8 mm in diam.; surface: light gray or with slightly yellowish olive tinge, smooth and slightly shiny; cortex: leathery; fertile verrucae: strongly convex, 1.1-1.6 mm across, often constricted at the base and almost peltate; Apothecia: developing in older verrucae; mazaedium: 0.20-0.35 mm in diam., flat to convex, black; thalline margin: thick, smooth; cortex: consisting of three layers: the outermost one: hyaline, paraplectenchymatous, 813 µm; middle one: 25-55 µm, well delimited, with small, irregular crystals; innermost layer: very irregular and with prosoplectenchymatous hyphae penetrating deep into the medulla; exciple: indistinct; hypothecium: light brown; central part of exciple 0.12-0.16 mm high, consisting of dark-colored hyphae; hymenium: 70-90 µm tall; asci: cylindrical, 33-38 x 5-6 µm when mature, 6-8 spored, with uniseriate spores; ascospores: when mature non-septate, spherical, (9-)13-16(-17) µm, with a somewhat fine and irregular ornamentation; semi-mature spores: sometimes appearing faintly striate; Pycnidia: immersed, spherical, 0.08-0.14 mm in diam., to elongated, black; Spot tests: thallus: K-, C-, KC+ rose-red, P-; mazaedium: K+ red, P+ yellow to red; Secondary metabolites: thallus containing 3-chlorodivaricatic acid; mazaedium containing norstictic and salazinic acids.; Substrate and ecology: on rocks along the coast; World distribution: Europe, Macaronesia, coastal California to British Columbia; Sonoran distribution: southern California, Baja California, Baja California, and Sinaloa.; Notes: See notes under T. santessonii for comparison of Thelomma mammosum with that species.