Peltigera didactyla

Thallus to 5 cm diam., rosette-shaped or consisting of a few coalescing lobes in small groups level with or sunk into the substratum; lobes small, 0.5–1 cm broad and 1–2 cm long, mouse-ear-shaped when young, becoming markedly ascending when fertile; upper surface finely grey-white tomentose especially towards the margins; when young, with laminal, rounded or irregular pale sorediose patches; soralia to 2.5 mm diam., discrete, later becoming confluent, filled with pale blue-grey to brown coarsely granular soredia, disappearing with the formation of apothecia; lower surface with distinct raised pale cream or faintly flesh-coloured to ochraceous veins, anastomosing with well-developed interstices; rhizines simple, downy, occasionally bottlebrush-like; apices often somewhat brush-shaped. Apothecia held vertically, ± oblong, saddle-shaped, red-brown or brown, delicately crenulate and denticulate at the margin. Ascospores 40–75 × 3.5–4.5 µm, 3- to 7-septate. Conidiomata rare; conidia 6–7.5 × 3–4 µm. Thallus with low concentrations of gyrophoric acid (C+ red) and methyl gyrophorate have been occasionally recorded in the soralia. No lichen products detected by TLC.

Can be confused with the more markedly horizontally spreading Peltigera rufescens, which has a thickly tomentose upper surface, often large and conspicuous apothecia, with inrolled and coarsely crenulate or denticulate thalline margin and dark branched ± fasciculate rhizines. Also with P. membranacea, which is larger and has a bullate upper surface. P. didactyla is unusual in that a juvenile sorediate state is superseded by a fertile non-sorediate state; as the thalli become fertile, the soralia are reduced to pale, non-sorediate scars that eventually disappear.