138 H. JONSSON 



ing scattered in the Corallina-Gigartina-be\t, without playing any 

 further part in the vegetation. 



The Leathesia-association is here mentioned in connection 

 with the Corallina-Gigartina-beh. The brown, spherical, cartilaginous 

 Leathesia difformis is found in abundance in this belt only, and I 

 prefer to mention it here rather than to place it in the Chorda- 

 association. The species occurs only as an epiphyte, not only on 

 Ahnfeltia but also on Corallina, Gigartina and Chondrus, and is often 

 wonderfully numerous. The size varies greatly, often the globules 

 may be rather large and, by their yellowish colour greatly alter 

 the appearance of the vegetation. It often almost entirely covers 

 rather extensive Corallineta. The species is a summer- form, which 

 does not occur in any quantity worthy of mention until the month 

 of June, flourishes during July and August, and disappears in Sep- 

 tember, at any rate in SW. Iceland. 



Small specimens of Leathesia occur also on Rhodymenia palmata. 



13. The Crustaceous alga-association. 



At low-water mark and in the lower littoral zone, there occurs 

 a crustaceous algal vegetation which may be considered a direct 

 continuation of the sublittoral crustaceous algal vegetation (see be- 

 low, p. 148). Here, the species are, for the most part, the same, e. g. 

 Lithoderma fatiscens and Lithothamnion compaction. Both these 

 species often have a somewhat considerable distribution in the lower 

 littoral zone. The Hildenbrandia-associalicm mentioned as existing 

 in the upper littoral zone also occurs here. Lithothamnion lo3ve and 

 Phymatolithon polymorphnm also occur in the lower littoral zone. 

 In addition, we have Ralfsia deusta which, in many places, forms 

 patches upon the bottom of pools in the lower littoral zone, and 

 plays rather an important role in the vegetation. 



Tide-Pools of the Lower Littoral Zone are most fre- 

 quently depressions which are either directly connected with the 

 part which is constantly submerged or separated from it by a ridge; 

 which may happen to be the case is unimportant as the period of 

 exposure in this part of the littoral zone is extremely short, as is 

 also seen from the fact that the vegetation in these depressions 

 consists of sublittoral associations, or more correctly, of stragglers 

 from them. Thus, Laminaria and Alaria species frequently occur in 



