advanced for this increase in size ; it is perhaps due to excessive 

 nutrition ; otherwise they seem normal. 



The majority of the representatives of this genus occur in the 

 temperate and north temperate zones, most frequently upon rocks 

 and trees. 



PL ATE 41. 



LECANORA HAGEXI Ach. 



1 . Natural size. 



2. Magnified. 



3. Section of apothecium. 



4. Section of thallus. 



t;. Paraphyses and spore-sac. 



6. Spores. 



7. Algae. 



4. ACAROSPORA Mass. Mem. Lich. 27. 1853. 

 Several authors have combined this genus with Lecanora, though 

 110 logical reason can be assigned for such a procedure, it having few, 

 if any characters in common with that genus. 



The thallus is almost wholly wanting in the lower forms ; in the 

 higher forms (A. chlorop/iana] it becomes almost distinctly foliose, 

 and lobed, particularly near the margin. In many respects the thal- 

 lus, as well as apothecia, resemble those of Haematomma. The fol- 

 lowing are the structural characters of the thallus : In most forms, 

 even the lower, there is a decided tendency toward a cortical structure ; 

 there is a well defined upper layer of cortical cells extending verti- 

 cally, which varies in thickness and also somewhat in structure ; in 

 all species the outermost cell-layers of the cortical tissue, above and 

 below, are lined with acid crystals to which the characteristic color 

 of the thallus is due, this color being dark in the lower forms and dis- 

 tinctly yellow in the higher. Upon these color-differences sub-genera 

 might be based, but it is, however, not probable that there are sufficient 

 differences to separate them as genera. The hyphal tissue in the 

 algal layer is also more or less cortical. The algae are Plcurococcns 

 vulgaris and tend to arrange themselves in vertical rows parallel to the 

 hyphal cells. In the higher species there is a distinct lower cortical 

 layer which resembles the upper cortical layer, though it is thinner ; 

 in the lower forms the semicortical medullary tissue gradually merges 

 into the rigid rhizoidal hyphae. 



The apothecia are quite variable in size and form, and in all 



