Fink: SoME Common Types oF LICHEN Formations 415 
of lichen formations of our northern prairies, viz., those of the 
bowlders. I shall give a list of the lichen species most commonly 
occurring in such formations and shall designate the formations 
thus : 
THE LeEcaNnorA ForMATIONS OF ExposED BOWLDERS 
Physcia stellaris (L.) Tuck. 
Physcta caesia (Hoffm.) Nyl. 
Placodium cerinum sideritis Tuck. 
Placodium vitellinum (Ehrh.) Naeg. & Hepp. 
Lecanora rubina (Vill.) Ach. 
Lecanora varia (Ehrh.) Nyl. 
Lecanora cinerea (L.) Sommer. 
Lecanora fuscata (Schrad.) Th. Fr. 
Rinodina sophodes (Ach.) Nyl. 
This short list includes oniy about one third or one fourth of 
the lichens that may commonly occur in the formation in a locality 
where it is well developed; but the few given may be looked for 
in any favorable locality in the northern United States, and the 
addition of other species would scarcely show any greater amount 
of variation as to adaptations. Indeed, it will be sufficient to con- 
sider the adaptations of the genera of the above list rather than 
those of each species. The species of Lecanora and Placodium are 
the most characteristic plants of such formations. The species 
found on exposed bowlders are nearly all strictly crustose and 
closely adnate, so that moisture is confined in the substratum 
below the thallus and readily absorbed. The larger species of 
Lecanora usually have well developed upper cortices for protection 
against excessive evaporation. The species of Ylacodium have upper 
cortices more or less developed, and some of the species have 
such structures below as well. The species of Physcta are foli- 
ose, but closely adnate; and those commonly found in these 
exposed formations have well developed cellular cortices on all 
sides. Kinodina has a small thallus, consisting of a tangle of 
hyphae enclosing the algal cells and protected only by the upper 
more or less horizontal layers of hyphae and the entangled dead 
algal cells. In general, those plants of such formations which have 
no upper cortices have very small thalli, while those having the 
