Porpidia tuberculosa (Sm.) Hertel & Knoph
Thallus crustose, cracked to areolate, areoles smooth or slightly rugose and sometimes minutely papillate, grey-bluish, cream-grey, often with orange sections, more or less covered with slightly eroded soralia, scattered or clustered, round or irregular, 0.1-1 mm diam., soredia farinose to granulose, pale grey to grey-bluish, sometimes scoured and leaving a grey-bluish scar, prothallus dark brown, mostly distinct in case of contiguous colonies. Apothecia rare or in limited number, in small clusters of 2-4, rounded, 0.5-1.5 mm diam., immersed in the thallus at first then sessile, disc flat to slightly convex, often with minute warts, black, sometimes tinged blue owing to pruine, exciple thick, black. Ascospore simple, ellipsoid, 12-19 x 7-9 µm. Photobiont: Chlorococcoid. Medulla and soralia: C-, K-, KC-, Pd-, I+ violet. Rather common, on siliceous rocks, walls, stones, etc. N.B. This species is extraordinarily variable, including within the same colony, and may be difficult to identify in the fiel: the morphs with eroded soralia potentially resembles to Lecidea lactea which is K+ yellow > red, and the morphs with large soredia resembling to Porpidia soredizodes, K+ yellow and Pd+ orange. Fertile forms may also be compared to Porpidia cinereoatra devoid of soredia.