86 H. JONSSON 



in all. Of these species some are common and others rare; some 

 are found exclusively in the upper littoral zone and others extend 

 further downwards. In order to show this more distinctly, each 

 group will here be dealt with separately and in detail. 



Rhodophycese. Of the 18 species mentioned 5 are common, 

 and the remaining 13 rarer. The following are the commonly dis- 

 tributed species: Bangia fuscopurpurea, which is found exclusively 

 in the upper littoral zone; Porphyra umbilicalis which is found also 

 in the lower littoral zone; Polysiphonia fastigiata which occurs ex- 

 clusively on Ascophyllum nodosum in the upper littoral zone; Rho- 

 dochorton Rothii which extends to a depth of 10 metres, and Hilden- 

 brandia rosea which extends to a depth of 5 metres. The three 

 first-named species are common in places exposed to the light in 

 the littoral zone; Rhodochorton and Hildenbrandia , on the other 

 hand, occur most frequently as an undergrowth, or in shady clefts 

 (Rhodochorton), and at the bottom of pools (Hildenbrandia). These 

 two species are met with more rarely in places exposed to the 

 light. The 13 species which occur in the upper littoral zone, but 

 which must be called somewhat rare, are there shade-loving, and 

 are then found either as an undergrowth or in shady clefts and 

 depressions. Some are epiphytes, and are then protected against de- 

 siccation by the host-plant. The greater number of these 13 species 

 are common in the lower littoral zone, and the 3 Ceramzum-species 

 which are recorded exclusively from the upper littoral zone may 

 doubtless be expected to occur also in the lower littoral zone. 

 Conchocelis does not occur in the lower littoral zone but is found 

 at a depth of 6 — 35 metres. 



Phseophycese. Of brown algae the Fucaceaz play the most 

 important role. The species are few in number, but are of social 

 growth, and occur in such quantities that they comprise by far the 

 greater portion of the bulk of the plants in the upper littoral zone. 

 Of the 18 species mentioned above, 12 are commonly distributed, 

 and 6 are more rare, in the upper littoral zone. Of the 12 common 

 species, 9 are found exclusively in the upper littoral zone, viz., 6 

 species of Fucaceat, Sphacelaria britannica, Ectocarpus tomenlosus 

 and Ralfsia clavata; Pylaiella littoralis, Elachista fucicola and Isth- 

 moplea, on the other hand, range to a depth of about 10 metres. 

 Of the rarer species, Petroderma is found exclusively in the upper 

 littoral zone, Ralfsia verrucosa is most frequent in the upper littoral 

 zone, but is also met with in the lower littoral zone, as an epi- 



