96 Scientific Proceedings, Royal Dublin Society. 



proseckoides, and Licliina eonfinis, whose natural habitat seems to be the rocks 

 about high-water mark, extend their range upwards. 



This subvegetation occurs in more or less distinct zones, in the following 

 order, descending towards the sea : — 



Parmelias. Physcia parietina. 



Physcia aquila. Crustaceous species. 



The Parmelias are abundant on Howth, and extend up to the very 

 summit of the headland. They are usually found on the most weathered 

 rocks, in situations where they are sure of a fair supply of moisture, and 

 where there is shelter from severe winds. Along the upper edge of the cliffs 

 tliey are often associated with the two mosses, Grimmia inaviUma and Weissia 

 rupestris. Here they seem to have reached their ordinary seaward limit ; 

 but the shelter and protection afforded by the Ramalina growths enable 

 them to penetrate to lower levels, and in moist shady places they may even 

 reach tlie shore. Under these conditions they are frequently accompanied 

 by the two mosses mentioned above. 



The following are the species of Parmelia most often met with amongst 

 the Eamalinas of the Howth coasts : — 



Parmelia conspersa. Parmelia perlata. 



Mougeotii. saxatilis. 



prolixa. omphalodes. 



fuliglnosa. physodes. 



The species in the first list are of common occurrence all round 

 the headland, but are much more abundant along the south-west 

 coast. Those in the second list, being more alpine in character, keep to 

 higher levels on the sunny south-west coasts, but are the usual forms on 

 the eastern and nortliern coasts, where they are sometimes seen growing on 

 "rocks almost at sea-level. Parmelia prolixa is the most general of all the 

 species mentioned, and the one which penetrates furthest towards the sea. 

 It does not often grow associated with any other species, but forms large 

 stretches of pure brownish-green growths, fertile and vigorous, on the sloping 

 surfaces of the rocks, mainly on those with an easterly incline and in sheltered 

 and rather shady situations. It is also common on the bare flat tops of tlie low 

 cliffs of the south-west coast, just above the shaggy growths of Ramalina 

 which fringe their outlines, and is here often intermixed witli tufts of the 

 two mosses already mentioned. On sloping rocks Parmelia prolixa covers 

 bare spaces amongst the Ramalinas, and may be seen frequently competing 

 with them for room. Where the Parmelia has established itself only a few 

 poor fronds of Ramalina are to be seen here and there sticking up through 



