LICHENOLOGY OF ICELAND 143 
The subterranean thallus normally is free of gonidia (barring 
foreign-gonidia). It may be very strongly developed, and it pushes 
its way down among the particles of soil, which may gradually 
become entirely enclosed by its hyphæ. I have sometimes observed 
shapeless enclosed lumps of black humus (Lecidea decolorans, Bi- 
limbia sabuletorum (D. Lik. @., pl. 4, fig. 15), Bacidia citrinella), some- 
times organic remains with the cell-structure preserved (Lecidea de- 
colorans), and sometimes grains of mineral matter (Buellia scabrosa). 
In no case has it been possible to demonstrate whether solution 
takes place by the agency of the lichen-hyphæ. It is almost incom- 
prehensible that something of this kind should not happen, but it 
has not been proved. It is possible that what is set free of the 
enclosed organic remains, or of the mineral grains by purely che- 
mical decomposition, suffices for the lichens. 
In some few cases (Lecidea decolorans (D. Lik. Ø., pl. 10, fig. 53, c), 
Pannaria brunnea) 1 found, enclosed in the subterranean thallus, an 
undetermined species of green algæ. The gonidia were dead and 
decoloured, but the lichen-hyphæ had not sent haustoria into them 
(nor do they do so as a rule to their normal gonidia). The death 
of the gonidia was undoubtedly due to contact with the hyphe, 
and possibly some use had been made of their contents. The whole 
thing must be regarded as a Cephalodium-formation, a “hypogaean” 
cephalodium or perhaps a “pseudocephalodium.” 
About the mode of propagation of the crustaceous lichens very 
little is known. Ascospores, perhaps pycnoconidia, are probably their 
most common means of propagation, I have not observed soredia 
or detached portions of thallus in them, as in the fruticose lichens. 
It is a very interesting fact, that these means of propagation appear 
to be at any rate rare in the primitive, crustaceous lichens. 
Crustaceous lichens are very weak in competition with other 
plants, as these easily cover them over and exterminate them. They 
are most favourably situated in Iceland, and in other Arctic coun- 
tries; this will be discussed more fully below. 
Iceland has the following crustaceous earth-lichens: — 
Bacidia arceutina Ach. Baeomyces byssoides (L.) Th. Fr. 
—  caudata Nyl. = placophyllum Wahlenbg. 
—  flavo-virescens Dicks. Buellia badia Koerb. 
—  herbarum Hepp. —  parasema (Ach.) Th. Fr. 
— - milliaria Fr. —  scabrosa Koerb. 
— obscurata (Sm.) Th. Fr. Caloplaca cerina (Ehrh.) Th. Fr. 
—  squalescens Nyl. — Jungermanniæ(Vahl)Th.Fr. 
