16 52 Proceedings of the Royal Irish Academy. 



the latter ; but not infrequently F. vesiculosus forms a very wide zone reaching 

 from F. spiralis down to F. serratus, with Ascophyllum scattered amongst it, 

 or forming more or less of a band in the upper part. The reason for this is 

 explained later. At times one or more of the species may be absent, and this 

 may almost always be traced to lack of suitable means of attachment at the 

 appropriate level. In Achill Sound, for instance, the association often stops 

 abruptly with Ascophyllum or F. vesiculosus, owing to the absence of stones at 

 the level at which F. serratus should occur. In other places Pelvetia and 

 /■'. spiralis are only feebly developed owing to the lack, in the upper part of the 

 shore, of stones of sufficient size. 



Some idea of the width of Fucus association in Achill Sound will be gained 

 from the following readings : — 



A. Sloping shore near Achill Bridge with a narrow band of Fucaceae : — 



"Width nf zone. 



Pelvetia, ........ 2-3 feet. 



F. spiralis, ....... 5-6 ,, 



F. vesiculosus, ....... 10 ,, 



F. vesiculosus aud Ascophyllum mixed, . .10 ,, 



F. vesiculosus, ....... 10 ,, 



F. serratus, ....... 10 ,, 



B. Flat shore with a very wide Fucus association ; about 1 mile south of 

 Achill Bridge :— 



Width of zone. 



Pelveua, 5 yards. 



F. spiralis, ....... 10 ,, 



Ascophyllum, . . . . . . . 40 ,, 



Ascophyllum and F. vesiculosus, . . . 30 „ 



F. vesiculosus, . . . . • . 50 ,, 



F. vesiculosus, F. serratus, . . . . . 50 ,, 



The above figures represent fair average readings for sloping and flat 

 shores respectively, and serve to show that the horizontal range varies directly 

 with the amount of slope. On very flat shores even wider bands exist. At 

 Mulranny, for example, on a Mat stretch of rocks and stones at high-water 

 level, a growth of Pelvetia 80 yards wide was noted : this was followed by a 

 sudden increase in the fall, and the succeeding band of F. spiralis was only 3-4 

 yards in width. 



With regard to exposure, all the species composing the association can 

 tolerate the extreme shelter of the inner part of Clew Bay. On passing from 

 this region to the more open, beyond a slight change in form, there is no 



