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(Map updated: 11 August 2009)

This unmistakable, mustard-yellow ‘powder lichen’ of shaded, hard limestone underhangs and crevices can be told by its colour, lobe-like margins and spot tests. It can form thalli many centimetres wide or smaller fragments of a few millimetres or so where it is starting to become established. A white medulla may be seen where the surface is broken. Reactions: K+ purple. Northern half of Ireland, much rarer in the south.

Key characteristics

• Thallus mustard yellow, of powdery granules above a white medulla
• Edges clearly lobed; K+ purple, on hard, shaded limestone.

Original text submitted by Vince J. Giavarini