J. SMITH ON THE GENERA OF FERNS. 171 



giferous, the one on its superior and the other on its inferior 

 side, (that is, on the sides facing each other,) and by their 

 parallel proximity the sori are confluent, the free margins of 

 the two indusia conniving and are even imbricate ; at length 

 opening as it were by a central longitudinal fissure, and 

 therefore in appearance constituting only one sorus, this 

 structure characterizes the genus Scolopendrmm of Sir J. E. 

 Smith. Besides these three forms of sori, there are a num- 

 ber of species which have the ultimate divisions of their 

 fronds narrow, and producing only one sorus on each lacinia 

 or segment, which solitary sorus is all that characterizes the 

 genus Darea of Willdenow, but which I consider as only 

 forming a group of true Asplenium. The genus Acropteris 

 of Link is founded upon three small species having forked or 

 digitate fronds, the segments or laciniae being linear and nar- 

 row, so that only two nearly parallel veins are produced from 

 the base of each, and which are fertile on the sides facing 

 each other, with the indusia conniving as in Scolopendrium, 

 or even analogous to Blechnum; but judging from their 

 habit, these species must be viewed as the simple or reduced 

 form of an extensive group of Asplenium, which have prae- 

 morse forked segments, with the venules diverging in a 

 forked and digitate manner from the base or axis, the midrib 

 being generally evanescent or wanting. On viewing the 

 original species constituting the genus Allanlodla of Mr 

 Brown, they are found to agree so much in character with 

 the species which Roth and other authors have characterized 

 under Athyrium, that it appears to me necessary to associate 

 them together; and as regards Athyrium, as a genus, I must 

 confess that I cannot value it otherwise than as constituting 

 another group of Asplenium, the only character being in the 

 vaulted or cylindrical indusium, which is a form not uncom- 

 mon to many other species of Aspleniece. From these 

 observations it may be inferred, that the structure of the 

 sori and indusium affords but slight generic distinctions; 

 which is the more to be regretted, as the venation in the 

 greater mass of the species is uniformly direct and free : but 



