Dermatocarpon luridum  

Thallus repeatedly lobed, mat-forming, ± flaccid when moist; lobes 5-15 mm wide, 150-400 µm thick, often convex especially in the centre of the mat, attached by scattered holdfasts, rather thicker (to ca 650 µm) where ascomata are present. Upper surface epruinose, but often developing clusters of white crystals in dried collections, grey to pale brown, rarely dark brown in exposed places, bright green when wet. Lower surface smooth, pale brown, without rhizines. Upper cortex of small angular cells, with some brown pigment near the surface. Epinecral layer thin, comprising compressed cells. Medulla composed of filamentous hyphae, I+ red (in section, best seen using Melzer’s reagent). Lower cortex composed of of anticlinal rows of thick-walled cells, the inner cells larger than the outer, with brown pigment at the surface.Isidia and soredia absent. Photobiont a green alga (Diplosphaera).
Anamorph: no information available.
Teleomorph: ascomata perithecia, 500-600 µm diam., ± globose, immersed in the thallus, the ostiole inconspicuous. Peridium to 40 µm thick, composed of multiple layers of strongly flattened angular cells. Interascal tissue absent, but well-developed periphyses line the ostiole. Asci 80-95 x 19-24 µm, clavate, with a fairly long tapering stalk, the apex obtuse to rounded, not clearly fissitunicate, thin-walled, without clearly defined apical structures, 8-spored. Ascospores (10-) 12-19 (-23) × 5-8 μm, ellipsoidal to fusiform-ellipsoidal, sometimes inaequilateral, hyaline, aseptate, thin- and smooth-walled, without an epispore, gelatinous sheath or appendages.