276 THE ESSEX NATURALIST.
it would not be advisable to discard altogether the custom of
grouping lichens as corticolous, saxicolous, and terricolous,
when dealing with them ecologically. The terms lack precision.
A very common lichen of the rocky sea coast, Ramalina siliquosa,
is not there solely because of the chemical nature of the rock
on which it grows, but because the rock is in close proximity
to the sea.
Some lichens require to be immersed in sea water for a
lengthened period during each tide.; others take up a position
within reach of spring-tides only, while a group growing above the
level of the spring-tide needs the salt spray of the breaking waves,
and still another group, although growing in proximity to the
sea, is so situated that the plants are beyond the too frequent
reach of salt water, as is the case with the Ramalina just referred
to.
Lichens that grow within the tidal flow give to the rocks a
black mantle, consisting mostly of Verrucaria maura and V.
mucosa.. In a report (1911) by the Committee of the Clare
Island Survey, A. Lorrain Smith writes "The rocks border-
ing the sea and the great cliffs of the north-west shore of the Island
are black with an unbroken growth of V. maura." In another
part of the same report (1912), devoted to Algae, A. D. Cotton
gives a description of a plant association which he names the
Hildenbrantia-Verrucaria association, after the dominant sea-
weed and lichen, which, though similar in growth form, differ
in colour, the sea-weed being dark reddish-brown, whilst the
lichen is almost black (27).
Although Cotton reported on Algae he found it necessary
to include (in addition to the Verrucaria) two other lichens,
Lichina pygmaea and L. confinis, in his tabulated lists of algal
associations and other communities composing the rocky-shore
formation in the Clare Island area. The first of these occurs
between the tide marks, but the second is always above high-
tide level. There is a distinct zonation in the distribution of
these plants. "The respective zones sometimes approach each
other closely, but never overlap." He remarks that in the
absence of Fucus, Lichina pygmaea forms a useful means of
determining tide levels.
Lilian Lyle (29) notes that Lichina pygmaea on rocks and
boulders also forms whole nurseries for sporelings of Fucus.