Thallus immersed, white to pale grey, sometimes green-tinged in shade, not cracked, usually matt; often with dark lines between contiguous thalli. Anamorph: not known. Teleomorph: ascomata perithecia, almost completely immersed, in well-defined pits in rock. Involucrellum 0.2-0.3 m diam., disc-shaped, spreading laterally but scarcely downwards, flat to slightly convex, in surface view with 3-6 fine cracks radiating from the ostiole, the perithecium apex often surrounded by a fine crack. Ascoma 200-400 µm diam., the peridium hyaline to brown. Interascal tissue absent, the ostiole lined with long periphyses. Asci 95-135 x 25-35 µm, clavate, quite long-stalked, thick-walled, the apex rounded with a rather broad shallow ocular chamber, 8-spored. Ascospores (18.5-) 21-29.5 (-32) × (10-) 10.5-13 (-14.5) μm, ellipsoidal, often slightly inaequilateral, hyaline, aseptate, very thin-walled, smooth, without an epispore, gelatinous sheath or appendages.
Description partly adapted from Orange et al. (2009). Recent studies show that this species should be interpreted in a more restricted sense, and probably the specimens imaged here (and most other records of B. baldensis in Britain) should be referred to B. parmigera.