XxXli INTRODUCTION 
impossible to place the less highly developed lecideine families 
first as they are already described in Part II. The lichens con- 
taining blue-green alge (Myxophycez) have been considered 
first ; those with a simple homoiomerous thallus being followed 
by those with a heteromerous thallus, reaching the highest 
development in the Peltigeracee and Stictaceze, which include 
genera with both types of gonidia, blue-green and bright-green. 
To avoid any break in the arrangement, the Parmeliacez, 
which are allied to the Stictacez, are placed next. The gonidia ° 
in this and the following families are bright-green. Then follow 
the Usneacex, and after that family the Physciacew, marked by 
the peculiar polarilocular spores. The genera of Physciacez 
range from fruticose to crustaceous forms. The Lecanoracee 
and other crustaceous families close the first group. 
The second great group of Cyclocarpinee includes those 
families in which the apothecia are without a thalline margin 
(biatorine or lecideine), the gonidia taking no part in the forma- 
tion of the fruit: they are thus more primitive phylogentically 
than the preceding. Beginning with the highest families of the 
group, we place first the Gyrophoracee with a well-developed 
foliose thallus, followed by the Cladoniacexe, the filamentous 
Coenogoniaceze—very poorly represented in northern countries— 
and finally the great family of Lecideacee with a crustaceous 
or, at most, minutely squamulose thallus. 
In subseries Graphidinee, the families have a crustaceous 
thallus, with the exception of Rocellacez, which is fruticose and 
highly developed, the gonidia are all Chlorophycez and mostly 
from the genus Trentepohlia. The arrangement depends on the 
character of the fruit. : 
Series IT., Pyrenocarpez, includes a small family of Pyreni- 
diacez with blue-green gonidia (Myscophyceace), the others 
all contain Chlorophycez. There is one family, Dermatocarpacee, 
which is foliose, the other families are all crustaceous and are 
arranged according to perithecial characters. They are derived 
from the Pyrenomycetes. 
The arrangement adopted has been partly necessitated by the 
order of publication. In the previous Monograph (by Crombie), 
the thallus rather than the fruit formed the basis of classifica- 
tion. If it had been possible to republish the whole work 
simultaneously, a better scientific grouping would have been to 
begin with the Pyrenocarpee, in which the fruit form is more 
