“On the stones and low shore rocks that lie just above the ordinary high-tide level Placodium lobulatum grows abundantly, covering the rocks with a continuous sheet of brilliant colour.” With these brightly coloured lichens are associated several with greyish thalli such as:

(3) The Lichina vegetation, and (4) The Verrucaria maura belt. These two communities are intermingled, and it will therefore be better to consider them together. There are only two species of Lichina on this or any other shore, L. pygmaea and L. confinis; the latter grows above the tide-level, and sometimes high up on the cliffs, where it is subject to only occasional showers of spray: it forms on the Howth coast a band of vegetation four to five inches wide above the Verrucaria belt. Lichina pygmaea occurs nearer the water, and therefore mixed with and below Verrucaria maura. Those three zones were first pointed out by Nylander[1189] at Pornic, where however they were all submerged at high tide.

Verrucaria maura is one of the most abundant lichens of our rocky coasts, and is reported from Spitzbergen in the North to Graham Land in the Antarctic. It grows well within the range of sea-spray, covering great stretches of boulders and rocks with its dull-black crustaceous thallus. At Howth it is submerged only by the highest spring tides. Though it is the dominant lichen on that beach, other species such as V. memnonia, V. prominula, and V. aquatilis form part of the association, and more rarely V. scotina along with Arthopyrenia halodytes, A. leptotera and A. halizoa.

(5) The belt of marine Verrucarias. This association includes the species that are submerged by the tide for a longer or shorter period each day. The dominant species are Verrucaria microspora, V. striatula and V. mucosa. Arthopyrenia halodytes is also abundant; A. halizoa and A. marina are more rarely represented. Among the plants of Fucus spiralis, Verrucaria mucosa, the most wide-spreading of these marine forms, is “very conspicuous as a dark-green, almost black, band of greasy appearance stretching along the shore.” When growing in the shade, the thallus is of a brighter green colour.

An examination[1190] of the west coast of Ireland yielded much the same results, but with a still higher “white belt” formed mainly of Lecanora parella and L. atra which covered the rocks lying above high-water mark, “giving them the appearance of having been whitewashed.” A more general association for the same position as regards the tide is given by Wheldon and Wilson[1191] on the coasts of Arran as:

A somewhat similar series of “formations” was determined by Sandstede[1192] on the coast of Rügen. On erratic granite boulders washed by the tide he found: