Two parasitic species of Placopyrenium (Verrucariaceae) from freshwater habitats in north-west Europe

Two species of Placopyrenium, P. cinereoatratum and P. formosum, occur on siliceous rocks at the margins of rivers and lakes. Placopyrenium cinereoatratum is sometimes parasitic on Staurothele fissa; R formosum is described as a new species, and is initially parasitic on Aspicilia aquatica.