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(Map updated: 11 August 2009)

This green-grey, rarely recorded species of shaded calcareous rocks and outcrops has a distinctive, white arachnoid prothallus. It is known to occupy a zone shaded during the summer by taller grasses and other vegetation (sometimes alongside rivers). The thallus is covered in pale-green soralia. Discs are pale red-brown to dark grey-brown; spores, 12-18ųm long, three-septate, fusiform-ellipsoid. No lichen substances present. In Ireland, the species occurs in the north and in County Mayo.

Key characteristics

• Separated from other Bacidia species by the three-sepate spores and from the sterile crust of Lecidella scabra by the C- reaction of the thallus
• Able to colonise stone in low-light conditions, often when the vegetation is at its peak.

Original text submitted by Vince J. Giavarini