LICHENOLOGY OF ICELAND 183 
known are the fell-fields of the plateaux; but others exist also, 
as for instance vast sandy tracts with drifting sand, both in the 
highlands and in the lowlands. We shall now consider these deserts 
more closely, leaving out those with a rock-substratum, which will 
be discussed elsewhere. 
According to the substratum we can divide the deserts into 
stony, gravelly, sandy and clayey deserts. A division according to 
the principles of plant-ecology, cannot be undertaken, as the vege- 
tation has not been sufficiently investigated, from a statistical point 
of view. 
Stony Deserts are the stone-covered ridges (holt) of the 
lowlands, and the talus of fallen blocks and débris (Urd) of 
the highlands. The lichens growing directly on the stone-substratum, 
do not concern us here, but between the stones on the ridges there 
grow as chasmophytes, Dryas octopetala, Thymus Serpyllum, Silene 
acaulis, Potentilla verna, Cerastium alpinum, Arabis petra, Saxifraga 
cespitosa, Juncus trifidus, Luzula spicata, Achimilla alpina, Poa glauca, 
Elyna Bellardi, and a number of less frequent species (according to 
Jønsson). Interspersed in fhe moss-carpets occur (according to 
Jönsson’s list in “Vegetationen paa Snefellsnes,’ p. 41) the fol- 
lowing species: — 
Cladonia rangiferina (podetia- wandering fruticose lichen). 
Thamnolia vermicularis = 
Cladonia uncialis = 
Sphærophorus coralloides — 
Cetraria aculeata = 
Sphærophorus fragilis — 
Cladonia pyxidata (hypothallus-wandering fruticose lichen). 
= cornucopioides — 
Cetraria islandica (erect foliaceous lichen). 
Peltigera canina (horizontal foliaceous lichen). 
= rufescens == 
— aphtosa = 
Jønsson does not mention having found any crustaceous lichens, 
therefore these will hardly occur in conspicuous abundance, whilst 
it may be expected that, on future investigations, some or other of 
the small, inconspicuous species may be found, at any rate on 
decaying moss. 
Jønsson also mentions the fact that here and there Cladonia 
and Spherophorus spp. may occur as dominants in a sub-vegetation 
of mosses, in addition to more sparsely occurring Graminee. 
The taluses of fallen blocks and débris (Urd) in the 
