GENERA OF FERNS. 665 
it is difficult to discriminate them even by their technical 
characters. This genus was first characterised by Mr. Brown 
as distinct from Cyathea, by the sori being furnished 
with a * Jacerated, often obsolete, indusium,’ and from 
Hemitelia by the sori of the latter being situated on the 
middle of the veins; but the species now forming true 
Hemitelie are distinguished by part of the venation being 
anastomose, which character, as I have already noticed, ex- 
cludes two of the original species, that are now placed by me 
in Alsophila. In the original definition of Alsophila, the 
position of the sori is stated to be axillary as in Cyathea, but 
I have before noticed that the sori in the latter genus are not 
always strictly axillary, and such I find is also the case with 
Alsophila, which circumstance favours the view I have taken 
in extending the character of this genus, thus including cer- 
tain species that exhibit both forked and simple veins, or 
sometimes all simple, the position of the sori in such being 
usually medial. The form of the indusium (when present) 
is also highly variable; in some it consists of a very small 
interior attached scale-like membrane, occasionally so indis- 
tinct that it is difficult to be seen; in others it is entirely 
wanting, its place being usually occupied by numerous articu- 
late hairs, that, like the indusium, rise from the base of the 
receptacle; while, in other species, the membranous indusium 
is so fully developed that its base nearly surrounds the base 
of the receptacle, almost forming a complete cup, as in 
Cyathea, differing only by the presence of an open sinus on 
the exterior side. This being the case, it becomes difficult 
to say that such species do not belong to Cyathea ; especially 
Since, sometimes, the indusium of the latter becomes much 
beten. Therefore, on viewing the great uniformity of 
habit in all the species, and the gradual modifications of 
structure in venation, position of the sori, and nature of the 
indusium, I have been induced to place the whole under 
Alsophila. A careful examination of a considerable number 
of species has enabled me to arrange them in groups, as 
follows :— 
