Pertusaria coccodes (Ach.) Nyl.

    Thallus moderately thick, smooth to verrucose, rimose or cracked, pale greyish, pale grey-greenish, pale brown-greenish, pale brown-yellowish, prothallus pale brown to brown-black, not or faintly zonate only towards the margin. Thallus covered with abundant isidia, < 0.5mm tall, forming crowded clusters, globose, ovoid more or less elongate, sometimes apparently stalked, not constricted at base, seldom coralloid (in the hollows of the bark), mostly greyish to pale brown-greyish, tips darker, fragile and leaving circular scars when broken. Apothecia very rare, 1-1.5 mm diam., at apices of globose warts, disc punctiform. Ascii 2-spored. Ascospores 100-180 x 30-60 µm. Thallus: C-, K+ yellow > slowly blood-red, KC- in general but sometimes KC+ with red smears when K is fresh and in high concentration, Pd+ bright yellow to orange. Mostly on bark of deciduous trees, seldom on smooth acid rocks near the seashore, or on chapel walls, see Pertusaria coccodes forme saxicole.

N.B. On rocks, not to be confused with Pertusaria pseudocorallina much more common of which the thallus is ochre-pink and isidia light brown, constricted at base, mace-like and with tips dark brown, almost blackish, scattered all over the surface and over the areoles, the thallus being (usually) K+ yellow rapidly turning to red, Pd+ yellow and KC+ yellow > red or in between.

N.B. Beware of Pertusaria coronata, a rare very similar species which differs mainly in its chemical reactions: K+ yellow-orange becoming brown-orange (never red), KC+ yellowish, P+ yellow-orange.