substrate: lichens, lichenized fungi | rock siliceous, siliciferous, acidic.
Thompson, J., 1997. American Arctic Lichens: The Microlichens.
Thallus white or gray-white, at first parasitic on Xaruhoria and Caloplaca, first showing as a paling of the host, then becoming areolate-chinky or farinaceous, later becoming independent. Apothecia to 0.6 mm broad, black, flat to convex, pruinose or not; margin thin, disappearing; exciple brown-black, more or less radiate; hypothecium dark brown to black-brown; epithecium brown; hymenium 70-80 μm, hyaline, not inspersed; paraphyses 2 μm, not branched or with some branching in upper part, tips capitate, 4-7 μm, brown; spores olive-brown, becoming brown, from 3-septate to weakly muriform, with some of the central cells with I longitudinal septum, 13-26 x 6-11 μm.
This species is at first parasitic, then becomes independent, on calcareous rocks in shaded overhangs and on old bones. It is circumpolar arctic and alpine and also occurs in Venezuela. In North America it ranges south to California and Arizona.
The relation between B. epipolia (Ach.) Mong. and B. nivalis badly needs study. The first is reported as having 3-septate (rarely muriform) spores and pruinose apothecia, the latter 3-septate to poorly muriform spores with the two types of spores within the same hymenial layer and with epruinose apothecia. Resolution of this puzzle requires the work of a monographer.