334
THE ESSEX NATURALIST
Clearly the most abundant species are Lecanora dispersa and
Catillaria chalybeia, both occurring throughout the range from 2"
to 20" above the ground. L. dispersa is not so abundant in the
top few inches; here L. expallens and Diploicia canescens as well
as Catillaria chalybeia are the most important species. Candelari-
ella is practically confined to the top 2". This species is almost
invariably present on the tops (horizontal surface) of these posts
and others of different heights, and its growth is perhaps favoured
either by the cross-cut surface of the wood or by deposition of
birds' droppings.
The occurrence of Rinodina demissa in this habitat is of
interest as this species normally grows on siliceous rocks. (It is
in fact abundant on old bricks deposited on the sea-walls in
various places round the island.) Some specimens on the posts
have a smoother, thicker thallus and resemble R. exigua (Ach.)
S. F. Gray, but most are exactly similar to those inhabiting the
bricks and are entirely typical of R. demissa.
Biatorina graniformis is relatively scarce on the S.W. side of
the posts, and is confined to the top few inches. It is, however,
much more abundant on the other sides, where it often extends
throughout the height of the 20" posts. This species is not,
according to the records, a very common British lichen, but is
abundant in this peculiar habitat on Skippers Island. It shows
considerable variability: usually the thallus is completely im-
mersed in the wood with only the black pycnidia or pycnidial
clusters protruding, in which condition it could easily be mistaken
for an unlichenized fungus; sometimes, however, usually at
places higher above the ground (? where the influence of the sea
is less) the thallus is better developed, and very occasionally
apothecia may be present.
Figure 2. Table showing the approximate percentage cover of lichens
on the S.W. side of oak posts in four regions from 0 in. to 40 in.
above the ground. + indicates that the species was merely pre-
sent but covered no appreciable area; — indicates absence.