LICHEN COMMUNITIES 383 



width all round the coast. In summer, the colour of these lichens is so 

 brilliant that the belt is easily recognized from a considerable distance." The 

 most abundant species occur mainly in the following order descending 

 towards the sea : 



1 . Xanthoria parietina. 4. Placodium decipiens. 



2. Placodium murorum. 5. Placodium lobulatum. 



3. Placodium tegularis. 



"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 : 



Lecanora prosechoides. Biatorina lenticiilaris. 



Lecanora umbrina. Rinodina exigua var. demissa. 



Lecanora Hageni. Opegrapha calcarea f. heteromorpha. 

 Rhizocarpon alboatrum. 



(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 1 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. promi- 

 nula, 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. 



1 Nylander 1861. 



