42 ON GENERA AND SPECIES. 



side. This, according to my view, accounts for the ring- 

 appearing not to be truly vertical, but it is to be observed 

 that the obliquity is not general, for in the figures of the 

 sporangia of eight Cyatheaceous genera in Hooker and 

 Bauer's " Genera Filicum," the ring is shown to be verti- 

 cal. I therefore follow Robert Brown in placing Cyathece 

 in Polypodiacece. 



The next systematic work to be noticed is that of 

 Professor Mettenius, of Leipzig, who, in 1 858, commenced 

 publishing a work entitled " Uber einige Farngattingen " 

 (" On Some Genera of Ferns "), of which five parts have 

 appeared. In order to explain this author's system of 

 classification of species, I will give a brief outline of the 

 manner in which he treats the genus Polypodium. He 

 enumerates, and mostly describes in full, 258 species of 

 this genus, including in it all the forms possessing puncti- 

 form, oval, or linear, naked sori, thus restoring it to nearly 

 the state in which it was left by Swartz, Willdenow, and 

 Sprengel, and even including in it the genus Grammitis of 

 those authors. His reason for placing so many species 

 under one genus is on account of the numerous interme- 

 diate or transition forms, which he says so pass into one 

 another that he finds it difficult to define any group of 

 species as a distinct genus in the manner that Presl and 

 others had done. Notwithstanding this, however, he finds 

 it quite possible to divide the genera into sections and sub- 

 sections, of which he gives an elaborate synoptical table. 

 He first divides the 268 species, according to their veins, 

 being free or anastomose. Those with free veins are 

 placed under four sections, and those with anastomose 

 veins in thirteen sections. These sections and sub-sections 

 (which are numerous) in most cases bear the names that 

 designate the genera of Presl and others, and consequently 



