April 13, 2010:
This is a relatively large and quite striking lichen whose thallus is bright green when wet but becomes duller and browner if dried out.
In Britain it is quite local, often found in upland areas usually overgrowing bryophytes on base-rich substrates. However, photographed here it was in old coppiced woodland close to sea-level on small moss-covered limestone boulders.
Cephalodia (large dark spots) can be seen on the surface of the thallus. These contain a cyanobacterium capable of nitrogen fixation but the presence of any fertile apothecia is rare. Other characteristics are the curly (crisped) margins of the lobes of the thallus which has a white underside with black hairy rhizines.
Tightly crisped lobes are well in evidence
The black rhizines show up well against the white underside of the thallus.
I'm grateful to Jeremy Roberts and David Clarke for details of this locality.