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 wide by 1-2 cm long, mouse-ear-shaped (concave) when young, becoming markedly ascending when fertile. Upper surface finely grey-white tomentose especially towards the margins; when young, laminal, rounded or irregular, pale, sorediose patches are present. 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. Folioles and schizidia absent. Lower surface with distinct, raised, pale cream or faintly flesh-coloured to ochraceous veins, anastomosing with well-developed interstices. Rhizines simple, downy, or bottlebrush-like, the apices often somewhat brush-shaped. Photobiont cyanobacteria (Nostoc).
Anamorph: Conidiomata pycnidia, rare. Conidia 6-7.5 × 3-4 μm.
Teleomorph: ascomata apothecia, held vertically, ± oblong, saddle-shaped, red-brown or brown, delicately crenulate and denticulate at the margin. Hymenium hyaline. Interascal tissue of unbranched thick-walled paraphyses, the apices hardly swollen and immersed in a reddish brown mucoid epithecial layer. Asci cylindrical, short-stalked, thick-walled and fissitunicate, the apex rounded with a K/I+ blue annulus, 8-spored. Ascospores 40-75 × 3.5-4.5 μm, narrowly fusiform, the ends rounded and sometimes slightly swollen, 3- to 7-septate, hyaline, thin-walled, smooth, without an epispore, gelatinous sheath or appendages.
Chemistry: thallus with low concentrations of gyrophoric acid (C+ red) and methyl gyrophorate has occasionally been recorded in the soralia. No lichen products detected by TLC.
Assessed by Woods & Coppins (2012) as of Least Concern. The species is widespread and reasonably common, occurring in a broad range of habitats.
The species can be confused with the markedly horizontally spreading Peltigera rufescens, and 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 P. didactyla is unusual in that a juvenile sorediate state is superseded by a fertile non-sorediate state; as the thalli become fertile, the soralia are reduced to pale, non-sorediate scars that eventually disappear.
Scattered throughout the British Isles. BLS map here.
Characteristic of recently disturbed sites, including cuttings, earth banks, roadsides, lawns, old soil in flower pots, urban wasteland, mine spoil heaps, quarries and grey dunes.