tudinal plane ; they are rarely formed diagonally to the longi- 

 tudinal axis (Collemaceae, Omphalaria) . Simple and septate 

 spores are not usually included in the same genus. Septate spores 

 again form genera according to the number and direction of the 

 septa. The form of the spores also determines genera; for in- 

 stance acicular and elliptical spores should not be included in the 

 same genus (ex. Biatora and Bacidia). The number of spores in 

 each spore-sac may determine genera, as in Acarospora and Lcca- 

 nora ; very rarely also the size of the spores and the structure of 

 the spore-wall. 



It must, however, be remembered that there are exceptions to any 

 rule. Colored and colorless spores may occur in the same genus ; 

 likewise simple and septate spores, etc. Nor must it be supposed that 

 the spore-characters alone determine the genera. We must, in addi- 

 tion, consider the development of the thallus, the apothecium, the 

 exciple, the color of the apothecial disk, the algae, and the color of 

 the thallus. A number of genera are based upon algal differences 

 {Sticta and Stictina^ Psoroma and Pannaria, and others). 



I have found no occasion to maintain subgenera. In the genus 

 Cladoilia it might prove convenient to form subgenera based upon 

 the development of the podetia ; such as those with comparatively 

 simple podetia as in C. cariosa, those with cup-bearing podetia as in 

 C. verticillata and those with much branched podetia as in C. ran- 

 giferina. Upon closer examination it will however be found that 

 it is difficult to draw the dividing lines and for that reason the sub- 

 divisions have not been made. 



Species Characters. At the very outset it must be admitted that 

 our knowledge of the life-history of the majority of lichen-individuals 

 is not sufficiently perfect to enable us to give the limitations of the 

 species. In other words no one has as yet made sufficiently reliable 

 observations to determine the possible variation of the individual 

 plant, and until this is accomplished it is in many cases impossible to 

 satisfactorily define species, much less to establish varieties, sub-va- 

 rieties, forms and sub-forms. It is, however, true that in the higher 

 forms of lichens the limitations of the species are fairly well deter- 

 mined. 



In general it may be stated that in the establishment of species all 

 known characters must be considered more or less. It is necessary 

 to enter into the consideration of the minute structure of the thallus, 



