MARINE ALGAL VEGETATION 



153 



stretches of the coast. It is the only species of this community 

 which forms a large, continuous and almost pure association, at 

 any rate over considerable areas. The upper part of the association 

 is, however, generally mixed with Delesseria simwsa, Ptilota, Odon- 

 thalia, Rhodophyllis and others, and then these species often occur 

 in such abundance that the community acquires its usual mixed 

 character: that is, several species occur dominantly side by side, 

 though no single species can be said to predominate. 



Among the remaining species of the community Rhodophyllis 

 dichotoma is often rather social. It is very common for Euthora 

 cristata to occur intermingled, but to judge from the dredgings it 

 seems to have a scattered growth. Ptilota pectinata may be of fairly 

 social growth in E. Iceland. Delesseria sanguinea and Polysiphonia 

 urceolata occur also in this community, the first mentioned ap- 

 pearing to be most frequent while, in a few places, the latter has 

 been found in abundance. 



Of the species with a scattered growth which belong to this 

 community may be mentioned: Lomentaria clavellosa (20 — 40 metres), 

 L. rosea (20 — 40) and Plocamium coccineum (20 metres), all in the 

 Vestmannaeyjar: and also Turner ella Pennyi and Omphalophyllum 

 uluaceum in E. Iceland. 



In many places the under-vegetation of the community is formed 

 of crustaceous species of Lithothamnion. 



Of the intermixed species, Desmarestia aculeata and D. viridis 

 are very frequent. In E. Iceland Chorda tomentosa has been found 

 intermingled in this community. In addition, Laminariaceoz of scat- 

 tered growth such as Laminaria hyperborea, L. digitata, L. saccharina, 

 Alaria esculenta, f. pinnata and Alaria Pylaii occur very frequently; 

 they are the outposts of the Laminariaeew-commumty. 



It has been previously mentioned that many of the members 

 of this community grow on Laminaria stems, and that in several 

 places the community forms, together with species of Desmarestia, 

 the second layer of the under-vegetation of the community of 

 Laminar iaceo?. 



The community grows both on somewhat exposed and also on 

 exposed coasts, and occurs both on a rocky and on a pebbly sub- 

 stratum, and even on sand. 



The semi-littoral Polysiphonia urceo/ata-association has much in 

 common with this community. 



Rosenvinge (G3) and, following him, Borgesen (12) call this 



