Arthonia fusca

The thallus of this diminutive species is variable and can either be immersed or comprise a scurfy crust that is greyish with an olive-brown hue. The apothecia are blackish, typically 0.2-0.4 mm in diameter, with 1-septate spores, 11-15(-18) x 4-7 µm. The species occurs both on calcareous and metalliferous substrates, such as metal-rich clinker and drip-lines below metal window grilles that contain copper. In West Glamorgan, to date it has only be recorded on the copper slag boulders at Penclawdd, but is likely to more widespread.