Porpidia macrocarpa (DC.) Hertel & Knoph.
Thallus thin, smooth to rimose, seldom thick, areolate to more or less verrucose, pale grey, grey-greenish, often partly covered with rusty spots, prothallus inconspicuous or wavy and blackish. Apothecia usually numerous, scattered over the thallus, sessile, to 3 mm diam., disc flat then convex and finally markedly convex, sometimes gyrose or with 'multiple' exciples, black, seldom powdered with grey pruine, exciple more or less wavy apparently dividing apothecia as if there were several exciples, thick (0.15-0.2 mm wide), persitent for a long time, black. Ascospores colourless, simple, ellipsoid, 16-20 x 5-12 µm. Photobiont: Chlorococcoid. Medulla: K+/- yellow, Pd+/- yellow-orange, I-. Not especially maritime but common on acid rocks. In the field, this species is very difficult to distinguish from Porpidia platycarpoides that is more coastal, K+ yellow to red and P+ yellow, and from Porpidia crustulata of which the apothecia are smaller an the exciple thinner. N.B. It is highly probable that many taxa are brought together under this name since this species is extremely variable.*