IOWA ACADEMY OF SCIENCES. 181 



This table affects comparisons thus far made in no way since 

 the Iowa locality, now under consideration, is a different one 

 than that previously used. In all the comparisons thus far 

 made, the lichens on the Saint Peter sandstone have been elim- 

 inated with those of other substrata occurring in only one of 

 the localities. Now, in the above table it will be noticed, that 

 the Iowa region has the advantage in the number of species not 

 common to both. Knowing what occurs in Iowa, I examined, 

 the Minnesota locality very carefully, and. the advantage is 

 apparently due to the more favorableconditions for lichen growth 

 in northern Iowa. The four species common to both regions 

 are doubtless distributed along the river between the two 

 localities wherever these rocks are exposed. How far north 

 the four species found only in the Iowa locality extend, and 

 how far south that found only in the Minnesota locality extends, 

 are questions of interest. Knowledge on this point might lead 

 to a modification of views just stated. 



Usnea barbata (L ) Fr. var. Hlrta Fr., is also confined to the 

 Saint Peter sandstone at the Minnesota locality, but not at the 

 Iowa one, though occurring on this formation there also. This 

 rock is apparently its most natural habitat in the regions con- 

 sidered, to which habitat it is confined in the one less favora- 

 ble to lichen development. Though, as in this instance, I have 

 omitted from the last table the species found on these rocks 

 and also on other substrata near by in one or both regions, 

 abruptness in floral change due to stress caused by change in 

 substrata is seldom better illustrated than in comparing the 

 lichens of the Saint Peter sandstone with those of other sub- 

 strata that happen to lie adjacent. The distribution of lichens 

 on this rock formation in Minnesota, Wisconsin, Iowa and 

 Illinois is worthy of careful study. Other questions of distri- 

 bution would be brought to light, illustrated by the species 

 here considered, and doubtless by several others not yet col- 

 lected on these rocks. 



From scattered statements in this paper, the inference might 

 be drawn that I should have given more prominence to differ- 

 ence of substrata in accounting for the difference in number of 

 lichens in the localities compared. Minneaoplis gains six spe- 

 cies on the Saint P ter sandstone, which is not found at Fay- 

 ette, and lacks six species, occurring at Fayette, because the 

 calcareous rocks seldom outcrop at the surface, and five species 

 because of scarcity < f calcareous earth. Possibly some allow- 



