LICHENOLOGY OF ICELAND 221 
the other name, but for the sake of synonymy it is necessary to 
have simple and easily definable conceptions, and this is best done 
by naming the association after the dominant plant-growth-form. 
With regard to lichens we will therefore employ as the prin- 
ciple of main division the grouping indicated above, viz. that of 
crustaceous, foliaceous and fruticose lichens, and, as far as possible, 
follow them on each rock-substratum. 
The Crustaceous-lichen-association is widely distributed 
on all kinds of basalt. Several types (formations) may be distin- 
guished, e. g mixed crustaceous-lichen-formations, Staurothele- 
formations, Caloplaca-formations and Verrucaria-formations. 
Mixed crustaceous-lichen-formations are widely distributed 
especially on the almost vertical faces of basalt rocks along the 
fjords. 
The plant-density is often rather slight, in that the individuals 
are not in contact with each other, i. e. they leave the rock-surface 
visible between them. In such places, therefore, there is no actual 
competition between the species, and the community is consequently 
analogous to the desert-vegetation of loose soil. 
In other places the plants may be closely in contact with each 
other, and struggle for space. In this case competition arises, where 
sometimes the one and sometimes the other plant predominates, 
but all the circumstances concerning this interesting struggle have 
not been investigated and are not known. 
Many interesting observations could undoubtedly be made as 
regards the frequency-number and mass-occurrence of the single 
species under different conditions, but all this requires both a long 
sojourn and also patient investigations on the spot. I presume, that 
among other things, we should thereby acquire a closer knowledge 
of the life-necessities of each species, and that we should be able 
to sub-divide the “mixed crustaceous-lichen-associations” into per- 
haps as many formations as the number of the systematic species. 
But this the future must decide. 
To this association almost all the crustaceous lichens of Iceland 
must undoubtedly be referred, i. e. somewhat above 100 species. 
There are, however, some which occur repeatedly and which ought 
to be enumerated as characteristic of the association, viz. 
Lecanora cinerea. Lecanora intricata. 
— pallescens. — frustulosa. 
— atra. — sordida v. glaucoma. 
