Zwackhia prosodea (Ach.) Ertz

    Thallus crustose, thin to very thin, smooth or minutely cracked, pale grey, dull white, grey-whitish, sometimes pale brown-olive tinged with green, seldom brown. Apothecia lirellate, fissure narrow and width stable during maturation, numerous, rather sessile, as laying on the thallus surface, short, straight or curved, seldom branched, 1-3 x 0.2-0.3 mm, black without pruine. Ascii 8-spored, 80-110 x 15-20 µm according to our measures. Ascospores acicular, colourless, straight or very slightly curved, 8-14 septate, surrounded with a gelatinous perispore, 50-70 x 6-8 µm according to literature, 45-65 x 7-8 (10) µm according to our measures. Infrequent pycnidia with 3 x 1 µm pycnidiospores according to our measurements. Chemical spot tests negative. Not very rare oceanic species characteristic of the dry and not wet barks of the old oaks (Quercus), but also on the barks of the old Taxus, in particular of the enclosures of the vaults, also in edge of the sea on the old Cupressus; often (at least on Quercus) in the company of Schismatomma decolorans.

Cf. Zwackhia viridis very similar although its thallus is more brownish or olive and its apothecia half-immersed and growing on smooth and humid bark of young deciduous trees.

N.B. Probably less rare than reported in the inventories because by its macroscopic appearance this species was probably confused with Arthonia atra in the absence of microscopic control.