Verrucaria nigrescens Pers. var. nigrescens f. tectorum
Differs from the type by its thallus, thinner, by the presence of isidia, all over areoles. N.B. This form seems to be differently analysed by British authors describing 'green blastidia' on thalli. Cf. often confused with Verrucaria macrostoma f. furfuracea that is sorediate towards areole margin, with a thicker thallus markedly turning to green when moist. Intermediate forms may be found and their identification is rather tedious.