Thelotrema lepadinum (Ach.) Ach.

    Thallus crustose, more or less thick depending upon the bark substratum, smooth to felt-like or slightly rugose, whitish, cream, cream-greenish. Apothecia usually numerous, evenly distributed, at the top of prominent warts, volcano-like with a base of 1-2 mm diam., and a summit open like a crater of 0.2-0.5 mm diam., rim irregular, with at the crater bottom a white-cream membrane, torn at maturity, leaving small white fragments aside and clearly visible at the center a brown, brown-ochraceous disc. Ascii 120-140 x 30-40 µm according to our measurements, (1) 2-3-4 (8)-septed. Ascospores colourless, remaining colourless until maturity, lenghtly fusiform with pointed ends and 10-18 transverse and 1-5 longitudinal septa (aspect of grid or corn cob section or intestine in optical section), 60-140 x 12-28 µm according to literature, 70-125 x 15-30 µm according to our measurements. Photobiont : Trentepohlia. Chemical spot tests negative, although some specimens are sometimes K+ yellow turning to orange-red, see: Thelotrema lepadinum forme K+ orange. Strong oceanic trends, on bark of trees in humid oceanic woodlands, seldom on mossy and humid rocks of oceanic woodlands, but see: Thelotrema lepadinum forme saxicole des rochers humides and Thelotrema lepadinum forme saxicole des rochers secs.

N.B. According to the properties of the bark surface, smooth or rough, this species may show a variable appearance, notably concerning the look and distribution of apothecia but it is likely that this "species" is a collective species with several taxa.

N.B. Not to be confused with Pertusaria hymenea et Pertusaria leioplaca of which the apothecia and ascospores have a different structure.

_ If the thallus is greyish and the fertile warts smaller and few, see Thelotrema macrosporum whose spores are also larger and with rounded ends.