Trapelia coarctata (Sm.) M. Choisy

    Thallus crustose, continuous, more or less thick, granular but not squamulose, sometimes cracked of slightly areolate, whitish, pale greyish, sometimes with pink or green tinge, prothallus whitish, at times. Apothecia rather in low number, scattered all over the thallus, 0.2-0.8 mm diam., almost encrusted within the thallus, disc flat  to slightly convex or convex mostly when moist, reddish, brown-reddish turning dark-brown when mature, exciple thin, whitish, excluded when mature, often edged with small white fragments of thallus resulting from the surface breach due to the growth of apothecia. Ascii 8-spored, clavate to cylindrical, K/I-, 100-110 x 22-25 µm (according to our own measures). Ascospores colourless, simple, ellipsoid, external wall tender and easily torn in a K solution (potentially suggesting polarilocular ascopsores), 15-25 x 7-13 µm (19-22 x 9-12 µm according to our own measures). Photobiont: chlorococcoid. Thallus: C+ red, K-, KC+ red, Pd-. Not especially maritime, rather common on siliceous rocks, stone walls and stones of stony grounds.

N.B. In case of fragmented and squamulose thallus, see Trapelia involuta / glebulosa. 

N.B. See Trapelia elacista whose apothecia are whitish more or less globose and closed at first.