Knowles — The Maritime and Marine Lichens of Howth. 101 



concreta f. Westringii form an almost continuous covering on the rocks both 

 ahove and below the Cliff-walk, descending in some places nearly to sea- 

 level, and are very common as undergrowth among the Eamalinas of these 

 coasts. On the north coast Lecidea rivulosa and L. contigua are associated 

 with them, and all grow together on Ramalina-covered rooks. 



All the species mentioned above are invaders from higher levels, 

 working their way towards the shore. Those species that on sheltered shores 

 grow naturally below the liamalinas, such as Placodium murorum, P. lobulatum, 

 Rhizocarpon alboatrum, and Lecanora j}rosechoides, are often found in exposed 

 areas among the Ramalinas. 



Rhizocarpon albocdrum is very common on the Howth coasts, and some- 

 times covers considerable areas of rock unassociated witli any other species. 

 It is most general on the coarse quartzites ; and when it occurs as a pure 

 growtli is found on rather steep rocks, in shelter, between the orange belt and 

 the Ramalinas. Where the seas are rough, Rh. alhoatrum rises amongst the 

 Eamalina tufts, but on low sheltered shores it is very abundant in the upper 

 part of the orange belt, where it is often associated with Rinodina exigua, 

 Buellia niyriocarpa, Lecanora Hageni, L. nmhrina, and others. 



The subvegetation of the eastern and northern coasts is largely 

 composed of species that are common in alpine and upland regions. This 

 is in a great measure due to the steepness of the rocks and also to the colder 

 and drier conditions prevailing on these coasts. On the sunnier and moister 

 south and south-west coast the subvegetation is almost entirely made up of 

 lowland forms. Ou these coasts the rocks dip at rather low angles towards 

 the east and south-east, and often present steep precipitous faces towards the 

 west and south-west. The subvegetation is always more plentiful and there 

 is a greater preponderance of foliaceous species on the gentler slopes where 

 moisture lies longer and where there is shelter from the prevalent winds. 

 The composition of the rock and the amount of weathering of the surface 

 have also a great influence on the abundance and nature of the species. 

 The complete absence of crustaceous lichens from the steep walls of hard 

 quartzite at Eed Rocks that are covered with growths of Ramalina B. has 

 already been remarked upon. Where the cliffs are composed of softer rocks, 

 as the shales and grits at Hippy Hole and near the Needles, the steep 

 westerly sides support a thinner growth of Eamalinas, but are completely 

 clothed with a covering of crustaceous species, such as Buellias, Lecanoras, 

 and others. 



(2) The Orange Belt. 



Below the Ramalinas and between them and the sea several deep yellow 

 or orange-coloured lichens form a belt of varying width all round the coast. 



