Porpidia crustulata (Ach.) Hertel & KnophThallus crustose, endolithic or thinly epilithic, continuous to rimose-areolate, white, yellowish white, or pale grey, rarely ochre-coloured, sometimes delimited by a dark prothallus. Medulla white, I-. Apothecia lecideine, more or less round, sometimes clustered, slightly constricted at base, 0.3-0.8(-1.5) mm across, with a black, slightly concave to convex, usually epruinose disc, and a more or less persistent, usually entire, thin (0.07-0.17 mm wide), barely raised proper margin. Proper exciple black to dark brown in outer part, pale brown and K+ yellow (rarely K-) within, with 4-8 µm wide hyphae; epithecium brown or olivaceous brown; hymenium colourless, 60-90(-110) µm high, I+ blue; paraphyses weakly branched and anastomosing; hypothecium pale to usually dark brown. Asci 8-spored, elongate-clavate, with a thin, outer amyloid layer and a thickened tholus penetrated by a pore, the sides of which are strongly amyloid, <i>Porpidia</i>-type. Ascospores 1-celled, hyaline, ellipsoid, 10-18(-22) x 5-10 µm, not halonate. Pycnidia immersed. Conidia hyaline, bacilliform, 9-14 x c. 1 µm. Photobiont chlorococcoid. Spot tests: thallus K- or K+ yellow, C-, KC-, P- or P+ orange. Chemistry: stictic acid (major), constictic, norstictic and cryptostictic acids (all minor or trace), or without lichen substances. 