Description: Thallus crustose, fully endolithic, usually covering large surfaces, white to pale grey, often delimited by a dark prothalline line. Upper cortex (lithocortex) densely prosoplectenchymatous, of conglutinated hyphae intermingled with microcrystals; algal layer discontinuous, photobiont cells in clusters; medulla prosoplectenchymatous, sometimes with oil cells (macrospheroids) in the lower part. Perithecia flask-shaped, completely immersed in thallus and leaving pits in the rocks when they fall off. Involucrellum black, disc-shaped, first whole, later with radial cracks around the ostiole, narrower than the body of the perithecium (0.2-0.3 mm wide); excipulum 0.3-0.4 mm diam., mostly colourless at least at the base; hymenium colourless, K/I+ blue; hamathecium of periphyses in the ostiolar canal, and short periphysoids in the upper half of the perithecial cavity. Asci 8-spored, two-layered, fissitunicate, with a thin tholus reacting I-, Verrucaria-type. Ascospores 1-celled, hyaline, ellipsoid, often poorly developed, 18-24 x 8-12 µm. Photobiont chlorococcoid. Spot tests: all negative. Chemistry: no lichen substances.
Note: a mild-temperate species of compact calcareous rocks in natural, sheltered and shaded situations, with optimum in the submediterranean belt.