Species: Placynthium subradiatum
Photos: On limestone. Black Head, the Burren, W Ireland.
Blackish or dark olive-brown thallus forms +/- complete rosettes, narrow rings or arcs, dying from the centre. Margin with tightly packed, adpressed lobes, prothallus inconspicuous. Isidia are sparse or absent, apothecia are very rare.
Occurs on vertical faces of hard limestone rocks, usually on surfaces that are subject to periodic flushing. Rare, most Irish records from the Burren, Co. Clare. Chemical reactions negative.
Similar: P. nigrum. Widespread throughout Ireland.
Placynthium subradiatum. BLS Burren meeting, April 2009