Thallus superficial, grey-green (in shade) to dark brown, 70–500 µm thick, initially comprising discrete dorsiventrally flattened units which become crowded and overlapping, forming an uneven crust with sparse secondary cracks, sometimes overtopping each other to make a thick uneven crust. Thallus in section divided into units of variable size, 30–325 µm thick, polygonal and mostly coherent, or of somewhat rounded outline, composed of isodiametric to shortly oblong cells 5–9 x 4–8 µm in size, with numerous air spaces between the cells; upper (and sometimes also lower) surface of units with brown-pigmented walls. Prothallus not detected. Photobiont cells 5.5–9 x 4–6.5 µm.
Anamorph: not known.
Teleomorph: ascomata perithecia, 220–460 µm diam., forming low to moderately convex conic-hemispherical projections, sometimes with only the lower 20-30% of the ascoma height immersed in the thallus, the projecting part exposed or with an irregular thalline cover below, sometimes with the ascoma partly or almost completely overtopped by thallus units. Ostiole inconspicuous or appearing as a pale dot 20–40 µm diam., plane or slightly projecting. Involucrellum appressed to the peridium above, usually becoming broader below, rather steeply conic-hemispherical in outline, densely pigmented throughout or with a pale area adjacent to base of the peridium; dark brown to dark reddish brown, the ostiolar area often with dark green pigment. Ascoma 190–310 µm diam., the peridium hyaline or the outer layer pigmented throughout. Interascal tissue absent, periphyses 25–40 µm long. Asci clavate, I–, fissitunicate, the wall thickened above, ocular chamber usually present; dehiscence by extrusion of an endotunica to form a delicate rostrum, 8-spored. Ascospores arranged biseriately, (17–) 20.5–24 (–28) x (8–) 9–10.5 (–11.5) µm, 2–2.5 (–3) times as long as wide, ellipsoidal, hyaline, aseptate, without a gelatinous sheath or epispore.
Not formally assessed. Currently recorded only from three streams in North Wales; probably under-recorded, but potentially threatened (e.g. by inappropriate enviromental management).
This species resembles Verrucaria rosula in the uneven thallus composed of initially discrete units which coalesce and overlap with age. They are less finely crenulate than in V. rosula, and the surface is more coarsely uneven. In section, the thallus of V. nodosa is less clearly divided into small units than in V. rosula, and the cells frequently have air spaces between them. The involucrellum in V. nodosa is more uniform in thickness than in V. rosula.
Reported only from VC48 Merioneth and VC49 Caernarvon.
On rocks beside streams, unshaded or lightly shaded, in open woodland or unimproved grassland.