Lecidella stigmatea (Ach.) Hertel & Leuckert
Thallus crustose, often circular, to 10 cm diam., slightly rimose and cracked, sometimes slightly coarse and more or less verrucose towards the center, colour highly variable: from whitish, pale greyish to beige tinged green or brown. Apothecia, 0.5-1.5 mm diam., disc flat to slightly convex, black, brown-black tinged indigo or dark blue, exciple thick, distinct at least when young, bright, concolourous or slightly paler. Ascospores ellipsoid, simple, colourless, 10-16 x 6-9 µm. Photobiont: chlorococcoid. C-, K+/- yellow, Pd- (several chemotypes have been described). On calcareous or very slightly acid rocks and cement. Not especially maritime but common on harbour works and slightly acid rocks. See Lecidella carpathica of which the thallus is roughly granulose and apothecia markedly convex, on more acidic rocks. See also Clauzadea monticola of which the apothecia are more globose and reddish with exciple soon excluded.
Cf Lecidella anomaloides which occurs on vertical acid rocks and in the shade and whose proper rim of the apothecia is brown in section.