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 Biatora 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 Biatora decipiens (Ehrh.) Fr. has a very heavy cellular 

 cortex above, the Endocarpon has a well-developed cortex on 

 all sides while the Heppia 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 algge 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(EHRH.)NAEG. and HEPP. var. aurellum 

 ACH., C. 



Lecanora muralis (SCHREB.) SCHAER. var. versicolor FR. 



Lecanora subfusca (L.) ACH. 



Lecanora calcarea (L.) SOMMERF. var. contorta FR., C. 



Lecanora privigna (Acn.) NYL., C. 



Lecanora privigna (Acn.) NYL. var. pruinosa AUCT. 



Rinodina bischoffii (HEPP.) KBR. 



*Fink, B. 1. c., 297 



