Aspicilia calcarea

A chalky-white lichen with cracked-aerolate crust with a well-defined margin, which can give it the appearance of a bird dropping. The thallus varies in colour from the above to blue-grey. Apothecia with black discs.

Found on hard limestone often forming large circular patches. Some growths maybe 800 years old. Also found on urban substrates like on concrete and brick walls. Common and widespread.