731 



As to the Færoes, this formation has already been described 

 by Simmons (78, p. 252), who points out that it is widely distri- 

 buted here. 



To the Corrt//ma- formation I refer the following four charac- 

 teristic and universally destributed associations. 



The Monost roma Grevillei- Association. 



On horizontal rocks or in very shallow, httoral pools where 

 only very little water is left at low tide, Corallina will frequently 

 be fonnd at a height of a few feet above the lowest water mark, 

 covered by a very dense, low spinach-green matting of the Monostroma 

 Grevillei. Where Corallina sometimes is wanting, Monostroma Gre- 

 villei covers the rock itself. Its growth is so dense, that it is only 

 exceptionally intermingled with other species, such as Phyllitis fascia, 

 Scytosiphon lomentarius, Dumontia filiformis, etc. 



On account of its dense and gregarious growth, the Monostroma 

 is always kept moist by the great quantity of sea- water which it 

 can retain, even when it grows on a slightly sloping substratum. 



As Monostroma Grevillei is a spring and summer alga, this asso- 

 ciation disappears later in summer and the underlying Corallina- 

 vegetation is now clearly seen or is covered by other algæ. The 

 Monos/ro/na- association is very common along the coasts of the 

 Færoes and is found even in very exposed piaces, in spite of its 

 apparently slight power of resistance against the surf. 



The Acrosiphonia-Polysiphonia- Association. 



This association which is formed of species of Acrosiphonia, 

 especially A. albescens, and of Polysiphonia urceolata reaches from al- 

 most the lowest water mark to a few feet above it (part of this asso- 

 ciation is seen at the bottom of plate XVII). It grows on sloping rocks, 

 which it covers with a very dense matting, generally green at the top 

 and reddish-brown at the bottom, these two algæ frequently making 

 unmixed facies, an Acrosiphonia-facies uppermost and a Polysiphonia- 

 facies below it. They may however also be found intermingled. On 

 account of their very ample ramification ^ they are both able to 

 retain a great amount of sea-water at low tide, which protects them 

 from becoming dried up. 



^ The littoral form of Polysiphonia urceolata is far more ramified than the 

 piants that grow sublittorally (cf. Grau 37, p. 10). 



Botany of the Færoes. 47 



