416 Fink: Some Common Types oF LICHEN FORMATIONS 
cortical protection and support may attain a quite considerable 
size even in their exposed habitat. A large foliose lichen de- 
void of cellular cortex, as a Collema, could hardly exist in the 
formation. Finally among the lichens of such formations are a 
few foliose species with well-developed cortices on all sides, but 
still closely adnate ; and a much larger number of crustose forms 
which have upper cortices only or none, and which, though epilithic, 
are closely adnate and have small thalli. 
I have recorded from some Minnesota localities what I have 
called The Mixed formations of shaded bowlders. \ cannot dis- 
cuss these rarer formations within the limits of this paper, but 
merely wish to refer to them as being quite distinct from the ex- 
posed bowlder formations. Finally, before leaving this portion of 
the subject, I may state that the formations of extended rock ex- 
posures will not be found to differ greatly from those of the closely 
related bowlders, except where influenced by climatic conditions 
due to proximity to large bodies of water along the coasts or on 
the shores of the great lakes, or to high elevation in the moun- 
tainous regions. 
The three classes of lichen formations thus far discussed are 
the ones which may be most easily investigated in the field, and 
yet there are two more types which are so easily studied in many 
parts of our territory that I am disposed to give them some at- 
tention. I refer to the formations of exposed calcareous earth 
and to those of exposed calcareous pebbles or horizontally exposed 
limestone. Ina previous study, I have designated the first of these 
formations thus: Zhe Biatora decipiens formations of exposed cal- 
carcous earth. These formations are remarkably constant as to 
the elements composing them wherever I have observed them in 
Minnesota, Iowa and Illinois, and are likely to contain the species 
listed below, and few if any others, wherever well developed in the 
northern states. The list of species is: 
Heppia Despreauxii (Mont.) Tuck. 
Urceolaria scruposa (L.) Nyl. 
Biatora decipiens (Ehrh.) Fr. 
Biatora decipiens dealbata Auct. 
Biatora muscorum (Sw.) Tuck. 
Endocarpon hepaticum Ach. 
