288 



gions. This shows Ihat, as regards the occurrences, the agree- 

 ment between Europe and America is greater than that between 

 Europe and the other parts of the world mentioned above. 



In the table of p. 255-269 I have given a comparison of the forms ^ 

 which are "character ising" for the different samples, by which 

 1 mean forms which on account of their frequent occurrence in a 

 sample must be regarded as characteristic of it. Naturally, "cha- 

 racterising" forms do not occur in all samples, while on the 

 other hånd several such forms may be found in the same sample. 

 I have next, as regards the freshwaterr forms, divided the 

 samples (according to the nature of the localities) into three groups, 

 1) those from running water, 2) those from moors and moor- 

 land ditches, and 3) those from lak es and stagnant water. 

 but naturally the boundary lines between these three groups are not 

 always easily drawn, they will partly depend on individual judge- 

 ment. Also I have grouped those genera which contain "charac- 

 terising" forms according to their occurence in Zealand and in 

 Jutland^. The reason why I have not included Funen is that 

 the number of samples from that island is small in comparison 

 with that of the samples from Zealand and Jutland, so that the 

 percentages would become misleading. For the same reason I also 

 omitted the smaller islands. The numbers which are given under 

 the heading "Number of samples" are the numbers of the samples 

 in which I found "characterising" forms. Lastly, I wish to add, 

 that when in the following pages, of this part I describe forms of 

 genera as, e. g. "More common in Zealand than Jutland" or, "Only 

 in Zealand", the words apply to them exclusively as "charac- 

 terising" forms and have no reference to their occurrence taken 

 as a whole. 



The table on p. 257 demonstrates the following facts : — 

 More common in Zealand than Jutland, from „running water" ^ 

 are forms of the genera Achnanthes, Gomphonema, Meridion^ 

 Nitzschia, Fleurosigma, Stauroneis, Surirella and Synedra. 



* I use the term "form" to designate both species and varieties, 

 ^ Zealand and Jutland are in the following pages often abbreviated 

 to Z. and J. respectively. 



