150
THE ESSEX NATURALIST
This species is, in unpolluted areas, entirely acidophilus. However, in
polluted areas, it soon disappears from siliceous substrates, but manages
to survive by colonising calcareous substrates.
L. stigmatea (Ach.) Hertel & Leuckert
Frequent on concrete, asbestos-cement and mortar; rare on limestone.
51/68, 69, 79.
Limestone wall of All Saints' Church, Hutton (1972, PME-B); concrete
roofing of outbuilding, Norsey Farm, Billericay (1973, PME-B); mortar
on wall of St Mary's Church, Runwell (1974, PME-B); mortar between
bricks of railway bridge, near Dunton Hall (1974, PME-B); corrugated
asbestos-cement roofs, disused barracks, Langdon Hills (1974, PME-B);
concrete roofing tiles, One Tree Hill (1974, PME-B); asbestos-cement
garage and concrete path, 6 Fairview Avenue, Hutton (1974, PME-B).
Lepraria incana (L.) Ach.
Very common on brick, sandstone, mortar and trunks and boles of
deciduous trees. 51/68, 69, 78, 79, 52/70.
Occurs wherever conditions are damp and shaded enough, typically at
the bases of walls, gravestones and boles of trees, but also on mortar,
particularly within the urban areas.
Opegrapha atra Pers.
Very rare on Ulmus. 51/79.
At the very base of an old Ulmus stump in a hedgerow by St Mary's
Church, Runwell (1974, PME-B).
Parmelia revoluta Florke
Very rare on Salix. 52/70.
Bole of Salix beside stream, Galleywood Common (1972, IMH).
P. saxatilis (L.) Ach.
Very rare on bark. 51/69.
A few depauperate specimens on the bole of a garden espalier, Norsey
Road, Billericay (1972, IMH).
P. subaurifera Nyl.
Rare on Fraxinus. 52/70.
Fraxinus stumps, Thrift Wood, Bicknacre (1972, IMH).
P. sulcata Tayl.
Occasional on Salix and Fraxinus; rare on moss over sandstone and on
decorticate wood. 51/69, 52/70.
A single specimen on moss on horizontal surface of sandstone grave-
stone level with the ground, All Saints' Church, Hutton (1972, PME-B);
trunks of Salix, Galleywood Common (1972, IMH); Fraxinus stumps,
Thrift Wood, Bicknacre (1972, IMH); inclined mossy trunks of Salix
beside drained pond and on a decaying decorticate log, Norsey Farm,
Billericay (1973, PME-B).