Thallus dark green or dark greenish grey, thin (40–60 µm thick), smooth, uncracked, subgelatinous; cells weakly arranged in columns. Cortex with dull green pigment which is K-, HCl+ brownish, and then dull greenish but faded with addition of further K, or in part with brownish pigment. Medulla absent. Prothallus not seen.
Anamorph: not known.
Teleomorph: ascomata perithecia, forming low to moderate conic-hemispherical projections 200–420 µm diam., covered by a layer of thallus at first which is sometimes partly lost later. Perithecia 140–290 µm diam., the apex rounded to slightly flattened, black when the thalline layer has been eroded, the wall colourless or brown below. Ostiole with dull green pigment similar to that in the thallus. Involucrellum conical, or somewhat spreading at the sides and then curved down slightly. Asci broadly clavate, thick-walled especially at the apex, with rostrate dehiscence, (3–) 4 (–5)-spored. Ascospores arranged irregularly, (9–) 10·5–13·5 (–15) x (5·5–) 6–7 (–7·5) µm, length/width ratio (1·4–) 1·6–2·1 (–2·6), ellipsoidal, aseptate, hyaline, thin- and smooth-walled, without a perispore or gelatinous sheath.
Assessed by Woods & Coppins (2012) as Vulnerable (D2). Possibly under-recorded, but surveys in Wales have failed to detect further populations so it may be genuinely rare. The species may be affected by agricultural and forestry practices which increase silt deposition, and by changes in atmospheric chemistry which alter pH of the water in which it lives.
The only UK species of Verrucaria with four-spored asci. Also distinguished by the green, subgelatinous thallus (with green pigment), conical involucrellum and small ascospores.
In GB&I, reported from Wales (VC42 Brecon), Scotland (VC86 Stirling and VC90 Angus), and most recently England (VC62 NE Yorkshire).
On frequently immersed siliceous rock in streamlets and rivers.