160 OLA1 GALL01 



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. 0k.," 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 Funf stuck. 

 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 

 endolilhic 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 (Beck man n, 

 1 ( .H)7), for instance, Placodium (Lecanora) sa.ricola, Caloplaca mnrornm. 

 Dimehcna ore'ina, all of which have along their edges leaf-like 

 thallus-lobes, devoid of cortex on their under surface. 



In Placodinm 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. 0k., fig. 62, b). Beck- 

 in an n 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. Arthoniu ruderalis. 



fuscata. Bacidia caudata. 



Heppii. coprodes. 



