MARINE ALGAL VEGETATION 155 



forms of Mgagropila, and in my opinion they are outgrowths upon 

 old "blocks" of calcareous algae. The action of the current probably 

 loosens them, and they then roll about on the bottom. Undoubtedly 

 they can live fairly long in that condition, but if they roll about 

 much, they will surely by degrees go to pieces. 



This association recalls the semi-littoral Cora/Zzna-community, 

 particularly the Cora//zY?a-association. 



The Lithothamnion-associalion occurs in Greenland (Rosen- 

 vinge, 63) but not at the Faeroes (Borgesen, 12). 



18. The Community of Crustaceous Algae. 



The characteristic life-form of the crustaceous algae which is 

 so essentially different from that of the rest of the marine algae 

 seems to justify the idea that they all belong to one community. 

 With all of them the thallus is flat and, like the crustaceous lichens 

 on the rocks, adheres by the whole of its lower surface to the sub- 

 stratum. As the form and the manner of development of the thallus 

 in the different species are identical in their main features, I think 

 that the community may appropriately be named after the crusta- 

 ceous growth. 



The substratum of the community consists of rocks, pebbles, 

 mussel-shells and the like; also of other algae, especially species of 

 Laminaria. 



The community has a very large distribution both in a hori- 

 zontal and in a vertical direction, and possibly it is more particu- 

 larly members of this community which we may expect to find in 

 the vicinity of the absolute depth-limit of growth of algal vegeta- 

 tion. The community has already been mentioned as the under- 

 vegetation in the Laminariacece -community; it occurs also as an 

 undergrowth in communities which extend to a greater depth and 

 thus, partly as a dominant growth on the bottom and partly as 

 undergrowth, it reaches from the great depths right up to the limit 

 of low-tide. The semi-littoral and the littoral crustaceous alga- asso- 

 ciations should also be regarded as part of this community although, 

 for practical reasons, they have been dealt with earlier in this paper. 

 The community is pure, that is to say it is composed of only cru- 

 staceous algae ; there occur, it is true, various intermingled species, of 

 which the majority are Floridece, though some are Phceophycece, but these 

 I consider unessential and almost irrelevant to the crustaceous alga- 

 community proper. They have their homes in other communities, 



