Verrucaria nigrescens f. nigrescens  

Thallus superficial, dark brown to almost black, regularly cracked into areoles 200-800 µm wide, spreading with the edge usually well-defined. Areoles usually smooth, flat to slightly convex, sometimes with darker mottling or slightly raised irregular flattened radiating segments, the sides brown, with a jet black basal layer that extends to around half the height of the areole. Cortex poorly defined. Prothallus absent or inconspicuous due to strong melanization in the lower part of the thallus.
Anamorph: conidiomata not known.
Teleomorph: ascomata perithecia, half to three-quarters immersed in the thallus. Involucrellum 200-400 µm diam., hemispherical or extending to the base of the exciple. Exciple 150-250 µm diam., brown. Interascal tissue absent, but periphyses and periphysoides present. Hymenial gel hemiamyloid, I+ red (+ blue at very low concentrations of I), K/I + blue. Asci clavate, I–, fissitunicate, wall thickened above, ocular chamber usually present; dehiscence by extrusion of an endotunica to form a delicate rostrum; 8-spored. Ascospores arranged biseriately, (17-)19-27(-30) × 8-14 μm, ellipsoidal to slightly fusiform-ellipsoidal, hyaline, thin-walled, aseptate, without a gelatinous sheath or epispore.