MARINE ALGAL VEGETATION 133 



The Halosaccion-association also commences below the 

 lower boundary of the Fucus- belt, and extends from there to a 

 depth of about 5 metres below low-water mark. The species grows 

 very socially, entirely covering the rocky substratum upon which 

 it grows: it is very variable, and, while f. densa seems to extend 

 rather high up, in pools in the littoral zone, it is f. robusta which, 

 as a rule, reaches to the greatest depth. F. siibsimplex is very com- 

 mon just below the Fuczzs-belt, where it grows so densel) r that it 

 covers the substratum completely. It is very often interwoven with 

 byssal-threads of Mytilas edulis. In this form, as in the laid-bare f. 

 robusta, inflated shoots occur fairly frequently. As is the case with 

 Rhodymenia palmata, this species is very frequently decoloured in 

 the lower littoral zone; yet in both species the lower part of the 

 frond is, as a rule, reddish in colour. 



Of the epiphytes, Elachista fucicola is especially frequent, and 

 various other species may also occur closely applied to Halosaccion, 

 such as Porphyra miniata, Monostroma fascum, M. Grevillei and others. 

 At times the Monostroma species may be so abundant that they 

 may be said to play a distinct role as epiphytic vegetation. Among 

 the Halosaccion individuals, Dumontia filiformis, Chaitomorpha Mela- 

 gonium and others often occur growing scattered at the very limit 

 of low-tide. 



A similar Halosaccion- vegetation occurs in northern Nor- 

 way (F os lie, 18), and probably on the arctic coasts as well (Kj ell- 

 man, 36; Rosen vinge, 63). In the Faeroes the association is 

 poorly represented (Borgesen, 12, 13). 



11. The Polysiphonia urceolata-association. 



At and about low-water mark in the lower littoral zone, an 

 association is found consisting principally of excessively branching, 

 red algae which I will name after the species Polysiphonia urceolata, 

 which is dominant in the association. This vegetation frequently 

 occurs on a flat or convex substratum of rock (lava-substratum) 

 and often forms, especially in SW. Iceland, for example at Grotta 

 near Reykjavik, a broad belt around Corallina- pools, near low- 

 water mark. 



The species which occur in greatest abundance in this asso- 

 ciation are Polysiphonia urceolata, Cystoclonium purpurascens and 

 Rhodomela lycopodioides. 



The Polysiphonia urceolata-association is very luxuriant 





