Pertusaria lactescens Mudd.

    Thallus crustose, rather thin, smooth at first becoming rimose and areolate, areoles to 1.5-2 mm across, minutely granular in sections covered with minute papilla, globose isidia-shaped, 0.1 mm diam., pale cream-ochraceous, giving sections with sorediate aspect through erosion, thallus overall colour whitish, dull white-cream, pale white-ochre, prothallus inconspicuous, fimbriate, white. Seldom fertile and when fertile apothecia small, 2-5 per areole, disc brown blackish, but powdery and apparently white, exciple slightly wavy. Ascii usually 2-spored. Ascospores colourless, possibly brownish when mature, walls very thick, turning K+purple, 80-100 x 55-85 µm. Thallus: C-, K+ yellow > red, KC+ yellow > yellow, Pd+ golden yellow. Oceanic trends, on shaded weakly acid rocks and walls protected against the sun, notably on North wall of chapels. N.B. The sterile thallus is rather similar to Phlyctis argena of which the chemical reactions are similar (although slightly less bright and Pd+ orange). The latter grows on bark and seldom on rocks and stones, but isidia are more grey-greenish, which is not the case for Pertusaria lactescens (pale cream ochraceous). See Pertusaria excludens which shares the same chemical reactions, but with a thicker thallus, larger soredia, less isidia-like and usually growing on rocks near the seashore.