310 MARINE ECOLOGY 



wherever trickles of fresh water run down over the rocks to the sea, 

 and it is to be supposed that the lowered salinity is responsible for 

 this development. 



(2) A zone of Pelvetia canaliculata can be found on most shores 

 at about high-water mark and extending up as far as the spray goes. 



(3) Immediately below this there is often a zone of Fucus 

 spiralis or F. platycarpus. 



(4, 5) The next two belts vary in position, Ascophyllum nodosum 

 sometimes being the uppermost and in other places Fucus vesi- 

 culosus. Where both belts are present there is an intermediate zone 

 in which the two are mixed. 



(6) The lowest fucoid zone is commonly dominated by F. 

 serratus, but in certain areas it may merge at low-water mark into an 



(7) Himanthalia zone. 



At the same level as the Fucus serratus belt one may find that it is 

 partially replaced by communities of red algae, or that there is a 

 zone of such communities between the Fucus and Himanthalia 

 belts. There are three communities of this type which may be 

 frequently encountered in the different localities : 



(8) A Porphyra community with which Bangia and Urospora are 

 often associated. 



(9) A Laurencia community, the existence of which is frequently 

 marked in summer by the development of epiphytic forms such as 

 Cladophora and Foment aria. 



(10) A Gigartina community. 



On sheltered coasts Chrondrus crispus may occur at these low 

 levels. In the sublittoral there is commonly a bed of Laminaria 

 species in which L. digitata tends to be dominant near low-water 

 mark and L. Cloustoni farther down. L. saccharina appears in those 

 areas where the substrate is more or less sandy. 



The effect of the height of tidal rise upon the vertical extent of 

 the zonations is illustrated very well in Table XIII in which the 

 algal zones from four localities are all reduced to levels based on 

 mean low-water mark. The small range at Bembridge and Peveril 

 Point has resulted in a compression and overlapping of the zones, 

 whereas at Castletown, where the range is large, the zones overlap 

 but little and occupy a considerable vertical height. The level of 

 the upper zones in any locaHty is not entirely dependent upon the 



