156 H. JONSSON 



and sometimes, perhaps, are in the act of forming an upper vege- 

 tation ; sometimes they may be individuals which have "strayed" 

 beyond the real limits of their community. 



The community is divided into various associations, according 

 as to whether the one or the other of the species is dominant over 

 considerable areas. A widely distributed and typical Phymato- 

 lithon-poly morphum-association occurs, thus, in S. Iceland, and 

 Lithothamnion Lenormandi has a fairly social growth at Reykjavik. 

 L. Iceve and Clathromorphum compactum also form associations in 

 several places. Judging from the dredgings, Lithothamnion flavescens 

 and L. foecundnm have a more scattered growth while, on the other 

 hand, L. glaciate often occurs abundantly. Otherwise, it is very 

 common for the Lithothamnion species to grow intermingled with, 

 and at times upon, one another. The crustaceous, calcareous algse 

 form the greater part of this community in Iceland. The remaining 

 crustaceous species, such as Peijssonellia, Cruoria arctica and Litho- 

 derma fatiscens, are found more scattered, although the last-named 

 species forms associations in shallow water right up to the limit 

 of low-tide. These species must, however, be much more common 

 on the sea-bottom than is shown by the dredgings. It may be taken 

 for granted, also, that Petrocelis Hennedyi and Rhododermis parasitica 

 occur on a stony substratum in deeper water, seeing that they are 

 so common on the stems of Laminaria hyperborea at considerable 

 depths. Both Hildenbrandia and Petrocelis occur on a stony sub- 

 stratum at shallower depths, and also Ralfsia ovata. 



The species which has, with absolute certainty, been found 

 growing deepest is Lithothamnion lawe (see Part V), and it occurs 

 in masses at a depth of 88 metres, that is, it forms a Lithotham- 

 nionetum at this depth. Thus, of all the marine algal communities 

 in Iceland, this community extends deepest. 



A similar vegetation occurs in Greenland (Rosenvinge, 63, 

 p. 223), and in other places in the Arctic Sea (Kjellman, 36), but 

 in both these places it differs in the wider distribution and greater 

 luxuriancy of the Lithoderma /hfa'scens-association. In the Faeroes 

 (Borgesen, 12) the conditions seem to be somewhat similar, simi- 

 lar in any case to the conditions in S. and SW. Iceland , although 

 Lithoderma fatiscens seems to occur there more sparsely than in 

 Iceland. 



