Biscogniauxia marginata  

Stromata single or in small clusters, 3-6 (-8) mm diam x 3-4 mm thick, orbicular to broadly ellipsoidal but frequently irregular in shape, discoid to cupulate with distinctly raised margins; the surface carbonaceous, dark grey to dull black, sometimes slightly pruinose, when immature stromata hidden under an operculum-like ochraceous outer layer which dehisces as a whole. Stromatic tissue beneath the hymenial layer 2-3 mm thick, composed of variably bleached host tissue mixed with fungal hyphae, with a conspicuous melanized zone line.
Anamorph: Nodulisporium-like (Ju et al. 1998).
Teleomorph: ascomata perithecia, in a single layer, 350-500 µm diam and 500-800 µm tall, obovoid to tubular with an elongate ostiolar region, the peridium very thin-walled, grey-brown, composed of intertwined hyphal tissue. Ostioles deeply umbilicate, occasionally with two openings present within a single ostiolar crater. Interascal tissue of copious thin-walled unbranched paraphyses. Asci 115-135 x 12-14.5 µm, cylindrical, short-stalked, the apex obtuse, fairly thick-walled, with a discoid apical ring ca 5 µm diam. and 2 µm thick that stains blue in iodine, 8-spored. Ascospores arranged uniseriately, 13-15 x 10-12.5 µm, very dark brown, broadly ellipsoidal to subglobose, with a sigmoid germ slit extending the entire length of the spore, without a gelatinous sheath or appendages.

Map of known distribution in Great Britain and Ireland.