120 H. JONSSON 



In addition Ceramiela formed by Ceramium rubrum, and Cal- 

 lithamnioneta formed by Callithamnion Arbuscnla, occur distri- 

 buted in patches, especially in the Vestmannaeyjar. 



The following species, growing in a more scattered manner, 

 occur also in the under-vegetation of the Fucus-beli: — 



Delesseria alata. Plumaria elegans. 



Lithothamnia. 



The species of the under-vegetation thus belong principally to 

 the red algae; only two species being brown, one blue-green and 

 one being a lichen. The majority of the species belong to the shade- 

 vegetation of the littoral zone. 



4. The Enteromorpha-association. 



Of the Enteromorpha-species E. intestinalis is the most common 

 and the most variable. The variability, no doubt, principally de- 

 pends upon local conditions, possibly also, on the age of the species. 

 The typical form is extremely common in the upper littoral zone, 

 but occurs most luxuriantly in the water-filled depressions. As a 

 rule, the plants have a social growth, although they rarely cover 

 large areas. E. prolifera and E. clathrata, on the other hand, occur 

 in such abundance that it might be justifiable to speak of an asso- 

 ciation formed of branching Enter omorphas. These species are found 

 in greatest abundance in the middle littoral zone, often in empty 

 depressions in the lower part of the Fucus-belt proper, and may 

 sometimes extend beyond the boundary of the lower littoral zone, 

 where they then encounter semi-littoral associations, for example, 

 Dictyosiphonetiun, Chordarietum and others. While the typical form 

 of E. intestinalis seems to flourish during the spring, the branching 

 forms E. prolifera and E. clathrata are noticeable in the summer. 

 I have known these two species to occur predominantly in June, 

 July and August on the same substratum upon which Urospora 

 Wormskioldii was growing in great quantities in the month of May. 



Enteromorpha Linza is also a summer species, and, although 

 it forms in several places pure Enter omorpheta of lesser extent, plays 

 only an inferior role in the Fnferomorjo/ja-association. 



In E. Iceland, on the point between Seydisfjor5ur and LoSmundar- 

 fjordur, at the height of the lower part of the Fucus-belt and a 

 little lower, I found a Cn/oro/)nyceoe-vegetation which must be con- 

 sidered to belong most nearly to this association. A Monostro- 



