Porpidia crustulata (Ach.) Hertel et Knoph.
Thallus thin to evanescent, smooth to slightly areolate, cream, pale grey somewhat tinged orange on iron-rich rocks, prothallus frequently present, black. Apothecia abundant, 0.5-1.5 mm diam., scattered or in small clusters or aligned, disc flat becoming convex at the very end, black, glossy and sometimes slightly pruinose, exciple black, satiny, 0.05 mm wide in young apothecia, late excluded. Ascospores colourless, simple, ellipsoid, 10-17 x 5-9 µm. Photobiont: Chlorococcoid. Medulla : C-, K+/- yellow, Pd+/- yellow, I-. Rather common, on siliceous rocks on the seashore as well as inland, occasionally forming small colonies. This species is sometimes difficult to distinguish from Porpidia macrocarpa of which apothecia are larger with a thicker exciple. Also remind that some rusty morphs of Lecidea lithophila are equally I-, with brown matt apothecia when moist.*