15 32 Proceedings of the Royal Irish Academy. 



waves, especially when die weather at the time is bright and dry. Its 

 occurrence is therefore very different, not onlyat various seasons, but in different 

 years. In winter it is very abundant : but the critical period of the spring 

 will every year kill a greater part of it ; and on the duration and intensity 

 of this period depends to what degree that will take place. In summer, 

 for example, it occurs at Frederikshavn in some years only in small 

 quantity, while in others it forms extensive growths — as in the beginning 

 of August, 1902 " (l.c, p. 58). 



With regard to Clare Island, drying winds during a critical period of the 

 life-history no doubt largely influence the growth of the plants. The 

 association may occur annually as winter vegetation, and it may possibly 

 have been present in the early spring of 1910 ; but if this was the case, it 

 had completely disappeared by June, and was certainly absent in April, 1909. 



Comparison with other Countries. — An association corresponding to that 

 described has been noted in many countries. It is present in Norway, 

 Sweden, Greenland, Iceland, and the Faeroes, having been dealt with by 

 Kylin, Eosenvinge, Jonsson, and Borgesen respectively; and, as already 

 shown, it occurs in Denmark. The belt in these countries, as would be 

 expected, is found at a higher level than in Clare Island. It reaches, or 

 extends above, the high-water line in each case ; and in the exposed part of 

 the Faeroes it occurs as much as 30 feet above this level. In Greenland the 

 association differs floristically, and is termed by Eosenvinge the Monostroma 

 grocnlandicum association, after the dominant species. In the Baltic 

 (Gotland) Svedelius has described a similar vegetation, consisting entirely of 

 Urospora penicilliformis. 



Rhodymenia Association. 



An association of Ehodymenia similar to that described by Borgesen, and 

 noted by him as being widely spread on the Faeroes, occurs in our district, 

 though only to a limited extent. Borgesen stated that he had found no 

 reference to such an association, but that he expected it to be common in the 

 North Atlantic. In the Clare Island area it is but feebly developed ; but at 

 Dog's Bay, near Eoundstone, a very extensive growth was observed, showing 

 that the association exists on some parts of the west of Ireland, as indeed was 

 already known. On the island, patches of Ehodymenia occur in several 

 spots between Portlea and the lighthouse, and on the mainland at Eoonah and 

 at Old Head. 



Judging by the localities in which the plant was found, Ehodymenia 

 prefers a northern aspect, which would account for the complete absence of 

 its growth, as an association, on the south shore of Clare Island. At Old 



