ECOLOGY OF ROCKY COASTS 331 



tidal phenomena and metabolic activities of the plants it would be 

 unwise to use tide levels for this purpose. The supra-littoral does 

 not concern us here, though in passing we may note that it is 

 usually bare or occupied by terrestrial lichens. In most regions the 

 supra-littoral fringe is characterized by the presence of species of 

 Littorina or of aUied genera, i.e. Melaraphe in New Zealand and 

 Australia. If these are absent the region is occupied by Myxo- 

 phyceae and marine lichens and forms a black belt. The transition 

 to the major belt on the shore, the mid-littoral, is determined by 

 the upper limit of the barnacles, i.e. species of Balanus, Chatnae- 

 sipho, Chthalamus etc. In a given region, variations of exposure may 

 determine which particular species of barnacle provides the upper 

 limit. In most regions it is found necessary to subdivide this belt, 

 such subdivision being based upon the dominant organisms. Here 

 there is considerable variation throughout the world, although there 

 is some degree of uniformity throughout the cold waters of the 

 northern hemisphere on the one hand, and the cold waters of the 

 southern hemisphere on the other. At least a portion of the belt is 

 normally dominated by the barnacles. In the northern hemisphere 

 much of the mid-littoral is dominated by fucoids, e.g. Fucus vesicu- 

 losus, F. serratus, Ascophyllum. In the southern hemisphere oysters 

 and mussels may be dominant and towards the base of the region 

 there is a belt of Corallines or of Hormosira. The transition to the 

 next belt, the sub-littoral fringe, is determined by the upper limit 

 of the Laminarians, primarily in the northern hemisphere, or of 

 fucoids, e.g. Durvillea, Carpophyllum, Cystophora in the southern 

 hemisphere. The lower limit of this fringe is set just below extreme 

 low water-mark of spring tides, but at present our knowledge of 

 this fringe is not as good as that of our knowledge of the mid- 

 littoral and supra-littoral fringe. Since it is submerged for so much 

 of the time it is not an easy zone in which to work, and it is clear 

 that more work will be necessary before the lower limit of this 

 fringe can be satisfactorily settled. It differs from the supra-littoral 

 fringe in that contributions to its flora and fauna are made by 

 denizens of both the mid-littoral and sub-littoral proper. Despite 

 this there are species which appear to be restricted to the belt, 

 which itself possesses certain distinct environmental features, i.e. 

 it is a belt in which there are substantial periods of continuous sub- 

 mergence. Some workers deny the existence of the sub-Httoral 

 fringe in their regions and therefore do not include it in a gener- 



