332 THE ESSEX NATURALIST
tendency for C. rangiformis to be less common in the open
Plantago patches may be due simply to the greater exposure of
these patches.
2. Bark. The row of elms running north-south on Heron
Island affords one of the chief habitats for corticolous species.
Here the following occur: —
Parmelia subaurifera Nyl. o
P. fuliginosa Nyl. v. laetevirens (Flot.) Kichz. o
P. sulcata Tayl. f
P. borreri (Turn.) Turn.
(=P. dubia (Wulf.) Schaer.) o
Lecanora expallens (Pers.) Ach. o
L. conizaeoides Nyl. ex Cromb. a
Biatorina griffithii (Sm.) Massal. r
Xanthoria parietina (L.) Th. Pr. f
X. lychnea (Ach.) Th. Fr. 1 f
Physcia grisea (Lam.) Zahlbr. f
Ph. adscendens (Th. Fr.) Oliv. f
Ph. orbicularis (Neck.) Poetsch. f
Diploicia canescens (Dicks.) Massal. a
Buellia myriocarpa (DC.) de Not. a
Arthonia pruinata (Pers.) Steud. 1 a
(east side of trunk)
Opegrapha vulgata Ach. f
Lepraria aeruginosa (Wigg.) Sm. o
(shaded bases of trees)
On dead twigs on Holme Island, in addition to some of the
species recorded on elms, the following have also been found, but
all except Hypogymnia are scarce and represented by only small
specimens: —
Usnea comosa (Ach.) Rohl.
U. sp. (cf. U. hirta (L.) Wigg. emend Mot.)
Evernia prunastri (L.) Ach.
Hypogymnia physodes (L.) Wats.
By far the most abundant lichen on the twigs of both living
and dead trees and shrubs is Lecanora conizaeoides. This fact,
together with the scarcity of the larger fruticose lichens, indicates
a considerable degree of atmospheric pollution.
3. Wood. A characteristic lichen vegetation occupies old
wooden posts and stakes in saltings. Posts in various stages of
decomposition are present in and at the edge of saltings at a
number of places round the island, a particularly good series,
probably erected as part of the sea defences, lying along the
short stretch of old sea-wall running north-westwards from the
northern extremity of Heron Island. These are stakes of elm or
pine, nearly all about 20" high and 4" wide set at the foot of the
sea-wall at about mean high water mark where the phanerogamic
vegetation is dominated by Puccinellia maritima; they are