160 OLAF GALLOE 
have no equals when the surface of the substratum is fresh, i. e. 
has been recently bared or is non-disintegrated. They cannot, how- 
ever, live on very recently bared rock; a slight inorganic disinte- 
gration must first take place, and then they make their appearance. 
They themselves contribute towards disintegration whereby they 
prepare the substratum for other, more pretentious forms (foliaceous 
and fruticose lichens) and so bring about their own destruction, as 
mentioned in “Dan. Lik. Ok.,” p. 360. 
The endolithic crustaceous lichens appear to occur only 
on calcareous rocks. As an example may be mentioned Biatora 
immersa (Web.) Arn., which is exhaustively treated by Fünfstück. 
There is in this species a slightly developed epilithic thallus, con- 
taining gonidia, which at the base passes over into a more vigorous 
endolithic thallus, with a great abundance of oil-cells of various 
forms. There is evidently a certain connection between the great 
abundance of oil in the thallus, and the chemical nature of the 
substratum, especially its wealth of carbonates. This class and the 
calcareous lichens richer in gonidia, that is to say, on the whole, 
the endolithic and the epilithic species, are connected by a series 
of intermediate forms; and there is hardly any lichen which is 
endolithic in the sense that the whole of the thallus is hidden in 
the substratum and covered over by it. For the rest, there are 
many points in the natural history of the endolithic lichens, which 
still remain to be explained. With regard to special modes of pro- 
pagation, nothing is known. 
At the point of transition between crustaceous and foliaceous 
lichens there stands a group of “placoid” species (Beckmann, 
1907), for instance, Placodium (Lecanora) saxicola, Caloplaca murorum, 
Dimelena oreina, all of which have along their edges leaf-like 
thallus-lobes, devoid of cortex on their under surface. 
In Placodium saxicola there may occasionally be found an 
indication of a cortical layer on the under surface, when it is growing 
on a smooth, polished rock-surface (Dan. Lik. Ok., fig. 62, b). Beck- 
mann has shown that the species mentioned here may be propa- 
gated by the thallus-lobes becoming detached, and sprouting out 
into new individuals. 
Of crustaceous lichens Iceland has the following species: — 
Acarospora discreta. Arthonia ruderalis. 
— fuscata. Bacidia caudata. 
== Heppii. —  coprodes. 
