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 pygmcea 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 pygmcea forms a useful means of 
determining tide levels. 
Lilian Lyle (29) notes that Lichina pygmcea on rocks and 
boulders also forms whole nurseries for sporelings of Fucus> 
