Fink: LICHENS OF THE MINNESOTA VALLEY. 285 



Rinodina oreina (AcH.) MASS., C. 



Buellia spuria (SCHAER.) ARN., C. 



Buellia pullata TUCK., C. 



Buellia petraea (PLOT., KOERB.) TUCK. var. montagnaei 

 TUCK., C. 



Endocarpon miniatum (L.) SCHAER. var. complicatum 

 SCHAER., C. 



The formation on shaded rocks may be designated the Stauro- 

 thclc formation, after the prevailing genus. 



Staurothele formation of shaded or damp quartzile (Pipestone). 



Endocarpon miniatum (L.) SCHAER. 

 Staurothele umbrina (WAHL.) TUCK., C. 

 Staurothele drummondii TUCK., C. 



The lichen formations of the pipestone lying beside the quartz- 

 ite were studied to ascertain to what extent the difference in 

 chemical composition and hardness of the rocks would influence 

 the distribution of lichens, other ecologic factors being identical. 

 In the above table I have indicated species common to quartz- 

 ite and pipestone by (C.), and the table shows that only three 

 lichens were detected on the quartzite and not on the pipestone. 

 The following three, all growing in exposed places, were found 

 on the latter and not on the former. 



Placodium cinnabarrinum (Acn.) Auz. 



Placodium cerinum (HEDW.) NAEG. and HEPP. var. sideritis 

 TUCK. 



Lecanora muralis (SCHREB.) SCHAER. var. saxicola SCHAER. 



It is worthy of note that the differences are specific and that 

 the formations are identical generically. The appearance of a 

 certain plant in a particular set of ecological conditions is too 

 complicated a matter for exact explanation in many instances, 

 and I can offer no explanation as to why the few plants occur 

 on one kind of rock and not on the other. Possibly the specific 

 acid secreted by a particular species acts more readily on one 

 kind of rock than on the other, but more probably the cause is 

 other than this. Nor do I suppose that I have found, here or 

 in other formations to be considered below, all the lichens grow- 

 ing under a particular set of conditions. Yet the common ones 

 which give character to the various formations were doubtless 



