Description: Thallus crustose, fully endosubstratic, usually covering large surfaces, white to pale grey, without radial cracks around the perithecia. Cortex (lithocortex) 20-50 µm thick, densely prosoplectenchymatous, of conglutinated hyphae intermingled with microcrystals; algal layer discontinuous, the photobiont cells in clusters; medulla prosoplectenchymatous, with oil cells (macrospheroids) in lower part. Perithecia black, relatively numerous but less dense and less regularly arranged than in B. parmigera, completely immersed in thallus and leaving deep pits in the rock when they fall off. Involucrellum black, disc-shaped, with radial cracks around the ostiole, c. as large as exciple (0.2-0.3 mm); exciple black in a thin outer part, paler within, the pigmented part appearing as thin black line; hymenial gel hemiamyloid, I+ red (I+ blue at very low concentrations of I), K/I+ blue; hamathecium of periphyses in the ostiolar canal, and short periphysoids in the upper half of the perithecial cavity; paraphyses dissolving early. Asci 8-spored, two-layered, fissitunicate, with a thin tholus reacting I-, Verrucaria-type. Ascospores 1-celled, hyaline, ellipsoid, often poorly developed, c. 25 x 8-10 µm. Photobiont chlorococcoid. Spot tests: K-, C-, KC-, P-, UV-. Chemistry: without lichen substances.
Note: a poorly known member of the B. parmigera complex, apparently widespread in Mediterranean France, rarely distinguished elsewhere; to be looked for in Italy.