38 J. SMITH ON THE GENERA OF FERNS. 



II. An arrat^gement and definition of the Genera of Ferns, 



with observations on the affinities of each Genus ; by J. 

 Smith, A. L. S. 



[With Figures of the Venation of Ferns. Tabs. XVIII. XIX.] 



(Read before the Linnaean Society of London, February 18, April 7, and 

 June 16, 1840.) 



The vast number of new species of Ferns which have 

 of late years come to our knowledge, renders it necessary 

 that some additional characters, besides those hitherto used, 

 should be adopted for defining the limits of genera. The 

 necessity for this will be obvious, when it is stated that the 

 number of known species amounts to between 1600 and 1800, 

 of which probably about three-fourths have, till lately, been 

 placed under six genera; viz., Acrostichum, Poly podium, Pteris, 

 Asplenium, Aspidium, and Adiantum ; and, as might be ex- 

 pected in such extensive assemblages of species, many must 

 have been associated under the same genus, although having 

 no immediate affinity with each other, further than in some 

 arbitrary technical character. This has arisen from the generic 

 characters of Ferns having (with a few exceptions) been deriv- 

 ed solely from the form, position, and direction of the sori, in 

 conjunction with the sori being naked, or furnished with an 

 indusium of different forms. Such being the case (and having 

 formed a collection of Ferns amounting to about 1400 species) 

 I became anxious to arrange them in more natural groups, 

 and, with that view, I made a critical examination of the 

 whole. While so engaged, I was not inattentive to the obser- 

 vations of Mr Brown, in the Prodromus Flora; NovoeHollandia, 

 and also in WallicKs Plantce Asiaticce Rariores ; and follow- 

 ing up that gentleman's observations, as regards the position 

 of the sori on the veins, I found that the different forms o^ 

 venation afforded, in conjunction with the position of the sori, 

 very important characters, which I first made use of, in the 

 year 1835, for arranging the very extensive genus Polypodiuw, 

 and it was no little satisfaction to me when I found that my 



