Psora pseudorussellii

Psora pseudorussellii, the bordered scale, is a species of rock-dwelling squamulose lichen in the family Psoraceae. It forms patches of small, brown, scale-like structures with distinctive white margins on limestone and other calcareous rocks in eastern and southern North America, from southeastern Canada south into Mexico. The species was described in 1986 to separate it from similar material that had been confused with Psora russellii, and DNA studies have confirmed that it is restricted to North America, with European and Asian specimens previously identified as this species actually representing different taxa....read more on Wikipedia.