Pertusaria rupicola (Fr.) Harm.

   Thallus crustose, fairly thick, irregular surface more or less cracked or warted by numerous papillae, greenish-yellow in humid weather, livid ochraceous-yellow in dry weather, hypothallus generally not very prominent and sometimes absent, blackish. Occurs in two forms linked by intermediaries: a fertile form very warted with relatively rare apothecia, at the top of fruiting warts, irregular disc, brownish, often at the bottom of a cleft. Asci with 4 (8) spores. Colorless spores, simple with thick wall, broadly elliptical, 70-110 x 30-50 µm, see: Pertusaria rupicola forme fertile; a sterile form densely covered with subcylindrical "isidia like" 2 mm high and 0.5 mm wide, containing a pycnidium, see Pertusaria rupicola forma coralloidea. Photobiont: Chlorococcoid. Cortex: C+ orange, K+ yellow, KC+ yellow to orange, P-. Saxicolous species with coastal, suboceanic, and Mediterranean tendencies, growing on sloping or vertical rock faces with little sun exposure.

N.B. This species is very difficult to differentiate from the fertile form of Pertusaria pluripuncta, also P+ orange, particularly in western France. However, the latter species in its typical form is strictly coastal; its hypothallus is large and zoned, and its apothecia are not located at the top of fruiting warts but several per areole; records of Pertusaria rupicola in Brittany are probably erroneous.

N.B. Pertusaria flavicans, with similar coloration, is sorediate.