Physcia species are lichenized fungi belonging to the family Physciaceae. According to an estimate made in 2008, the genus contains about 70 species. At the macroscopic level, the thallus of these species can be foliose, small and adpressed, with a greenish-brown color that can coalesce to cover large areas. The lower surface is normally white and the upper surface, where soredia are found can have a yellowish-green color. The apothecia are frequent in most of the species, and they have a cup-like shape, black inside, with smooth, swollen margins.
These species have been extensively found in the United States in dusty environments, growing in all type of substrates (e.g. wood, metal). Our particular specimen was found in central Michigan, growing in the metallic part of a bridge over the Red Cedar River.
Elix, J. A. (1898). Physciaceae. Retrieved from https://www.anbg.gov.au/abrs/lichenlist/Physciaceae.pdf12.14.16