40 J. SMITH ON THE GENERA OF FERNS. 



trivance for assisting us in comparing and studying the enor- 

 mous multitude of species, which, without arrangement, our 

 minds could not embrace."* 



It sometimes happens, that although certain species agree 

 with others in venation and the character of their sori, &c., yet 

 they differ most essentially in what is termed their general 

 aspect or habit. In such cases, rather than form unnatural 

 combinations of species, I have choseii to keep them separate, 

 although even their technical distinctions be but slight; and 

 my reasons for so doing will be noticed under those genera 

 so circumstanced, and I am inclined to believe that in 

 these and other cases, distinctive characters are yet to be 

 discovered, by a careful examination of their more obscure 

 structure. 



In the following arrangement, the first division of import- 

 ance is characterized by the sporangia being destitute of, or 

 furnished with an elastic articulated ring, which surrounds the 

 sporangia, either in the direction of its attachment, (vertical) 

 or at a right- angle to the attachment (transverse) ; which char- 

 acter divides Ferns into three very unequal groups or sub- 

 orders ; the exannulate group not numbering more than 1 00 

 species, while about half that number may be said to compose 

 the truly transverse annulate group or Gleicheniacece ; there- 

 fore, the great mass of the species are characterized by the 

 presence of a vertical ring, and comprise what have been 

 termed true Ferns, or Polypodiaceoe. This group I have 

 divided into seven tribes, characterized by the different forms 

 and position of the sori and indusium, and although their 

 distinctive characters are in general well marked, yet many 

 instances occur where they are found forming transitions into 

 one another, through some approximating points of structure. 

 Before entering upon the definitions of these tribes and their 

 genera, it will be necessary to give a brief explanation of the 

 principal parts upon which their distinguishing characters are 

 founded, and in so doing I have to observe that it is extremely 

 difficult to give names that will define precisely all the various 



* Bentham. 



