Porina rivalis

Porina rivalis is a species of semi-aquatic, saxicolous (rock-dwelling), and crustose lichen in the family Trichotheliaceae. Found in Great Britain, it was formally described as a new species in 2015 by lichenologist Alan Orange. The type specimen was collected in Nant Walch near Llanwrtyd Wells (Breconshire) at an altitude of 230 m; here the lichen was found growing on stones submerged in a shaded stream. The species had been known previously from streams in Wales, but it had been incorrectly identified as Porina lectissima. Porina rivalis has also been recorded from streams in South-west England and North England. It has a thin brown thallus (20–70 μm thick) with prominent dark brown or black perithecia. Its ascospores are shaped like narrow ellipsoids, have three septa, and typically measure 13.0–17.5 by 4.0–5.5 μm.