Thallus immersed, the surface usually level with surrounding rock or slightly lower, pale grey to brown-grey, not cracked; sometimes thallus units reaching to surface of rock as minute, brown specks. Prothallus not seen, but the leading edge of thallus sometimes white, and the junction with neighbouring, conspecific thalli marked by a fine crack, groove, change in level, a whitish (rarely brown) line, or a row or double row of brown pycnidia.
Anamorph: conidiomata pycnidia, frequent, scattered, often numerous and easily seen, appearing as brown pits 60-100 µm diam. Conidia 3.5-4.5 × 0.8-1.2 μm, rod-shaped.
Teleomorph: ascomata perithecia, forming distinct projections 300-450 µm diam., black, not covered by the thallus, the apex flat or usually concave, leaving shallow pits in rock when decayed. Ostiole inconspicuous, or appearing as a slightly paler area 30-80 µm wide, or as a paler area up to 140 µm wide and occupying much of the concavity. Involucrellum well-developed, thick, flanking the upper half of exciple, which is pale below. Ascospores (15.5-) 16.5-20 (-23.5) × (6.5-) 7-8.5 (-9) μm, length/width ratio (1.9-) 2.0-2.7 (-3.4), cylindrical to ellipsoidal, aseptate, hyaline, thin-walled, smooth, often with a perispore to 1.5 μm thick.
Assessed by Woods & Coppins (2012) as of Least Concern. It has a scattered but broad distribution.
Distinguished by the immersed thallus, often dotted with pycnidia, the perithecia which typically have a concave apex and the usually somewhat oblong ascospores. Perhaps previously over-recorded for Verrucaria pinguicula which is very similar, but with the thallus usually at least thinly superficial and often cracked, the perithecia less obviously depressed above and without pycnidia. V. pinguicula is probably a more northern species than P. dufourii.
Widespread in limestone areas of the British Isles. BLS map here.
On hard limestones.