15 28 Proceedings oj the Royal Irish Academy. 



inconspicuous on the dark slaty rocks of Clare Island. It usually occurs a 

 short distance above Pelvetia, and extends up the cliffs on the south shore to 

 a height of 12-15 feet. In this position it is frequently drenched with spray, 

 though in summer it may be dry, except for rain, for weeks together. With 

 less exposure its vertical range is restricted. On the gentle slopes at 

 Kinnacorra it is only 3-4 feet above Pelvetia ; whilst on the flat shores of 

 Clew Bay and Achill Sound it hardly exceeds the neap-tide level, and is 

 regularly covered during the highest tides. This occasional flooding by spring- 

 tides in sheltered localities obviously takes the place of the soaking by spray 

 on open coasts. In Clew Bay and other places where rocks and boulders are 

 absent, the plant often occurs on fiat stones, and may even be traced as an 

 irregular band running along the shore. In Roundstone Bay the three plants, 

 L. confinis, P. canaliculata, and L. pygmaea were noted on large slabs, forming 

 luxuriant and well-marked zones, each species following the other as a band, 

 1-2 feet deep. 



The Lichina vegetation at Roscoff described by Joubin ('09) is evidently 

 very similar to that found in Ireland. Both species are present, and the 

 plants show a preference for exposed situations. 



Porphyra umbilicalis Association. 



Porphyra is one of those algae which occur as a definite association 

 on both exposed and sheltered shores. The form assumed by the plant is 

 very distinct in the two situations ; and it is curious that it has not been seen 

 as an association in localities offering intermediate conditions, although its 

 occurrence in small quantity is common enough. Another feature which 

 gives interest to the exposed Porphyra community is the diversity of its 

 bathymetric range in different countries, and even in different spots in the 

 same locality. For instance, in Clare Island the upper limit of the Porphyra 

 belt is almost always below the Pelvetia zone, whereas in the Faeroes the 

 greater part is normally above that belt, and on exposed shores it may occur 

 as much as 40-50 feet above high-water line. Doubtless surf and spray are 

 largely responsible for its presence in such positions, just as they are seen to 

 be directly responsible for minor variations on Clare Island. But this alone 

 does not account for all the differences that have been noted, and further 

 study is needed. 



On most parts of Clare Island a certain amount of Porphyra vegetation 

 is present ; in some it is extensive and forms broad or narrow bands ; in 

 others it is limited, and occurs only as small patches between other com- 

 munities. It prefers rocks of a moderate or rather steep slope, .but is also 



