Fink: LICHENS OF NORTHWESTERN MINNESOTA. 669 
rains, and it may be regarded as well established that such a 
sloping surface is best adapted to the development of the forma- 
tion. 
As to structural adaptations to an exposed and usually dry 
habitat, the lichens of the formation all have small thalli and 
are closely adnate. The rudimentary thallus of Bzatora mus- 
corum (Sw.) Tuck. has no cortex, and the somewhat better de- 
veloped one of the Urceolaria has only a poorly developed 
pseudo-cortex above. Of the larger and better developed thalli, 
that of Bratora decipiens (Ehrh.) Fr. has a very heavy cellular 
cortex above, the Hxdocarfon has a well-developed cortex on 
all sides while the Hepfpza is cellular throughout. In these 
larger thalli growing in exposed dry places, the cellular areas 
serve of course not only for support, but also for protection of 
the alge within and against excessive evaporation; nor must it 
be supposed that any of these thalli are large for a really large 
thallus is seldon seen in such a formation, and if present at all, 
should be considered an accident in plant distribution not to be 
recorded. The four better developed thalli are then only large 
in comparison with the other two and average about 3 to 6 mm. 
in diameter. The two rudimentary thalli of the formation are 
able to persist because very small and closely adnate. 
Comparing next the lichen formation of the calcareous peb- 
bles of the same area with the same two for the Minnesota and 
Iowa localities used above,* we find that the adding of another 
Minnesota locality still leaves the same five species common (C) 
to such formations for the two states. This third formation of 
the kind recorded below, establishes a general resemblance of 
such formations in widely separate localities. 
LECANORA CALCAREA CONTORTA LICHEN FORMATION OF EX- 
POSED LIMESTONE (LEAF HILLS). 
Placodium vitellinum (Euru.) Nare. and Hepp. var. aurellum 
Pen. C. 
Lecanora muralis (SCHREB.) SCHAER. var. versicolor FR. 
Lecanora subfusca (L.) Acu. 
Lecanora calcarea (L.) Sommerr. var. contorta Fr., C. 
Lecanora privigna (Acu.) Nyt., C. 
Lecanora privigna (Acu.) Nyw. var. pruinosa Aucrt. 
Rinodina bischoffii (Hrpp.) Kzr. 
*Fink, B. 1. c., 297 
