Thallus crustose, thin, rimose, white to pale grey, rimose-cracked, mostly 10 to 20 mm diam., bordered by a conspicuous black prothallus and often forming mosaics.

Unless identified by a recognised expert, a photo is required, and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

On hard, siliceous, rocks.

All year round.

Over-recorded in the past owing to confusion with Buellia aethalea.  It is mainly recorded from the north and west of Britain.

Status in Leicestershire and Rutland not known.