PTERIDOPHYTA 281 



they occur in numbers enough to be a conspicuous feature of the vege- 

 tation. The general characters of the family have already been dis- 

 cussed, and the family is an extremely natural one. The differences 

 between the genera and species are of secondary importance, based 

 upon the position of the sori, the character of the indusium, etc. 

 (Figs. 247, 248). About one hundred and sixty species occur within 

 the United States. 



