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HISTORY OF BRITISH FERNS. 
Genus XVL HYMENOPHYLLUM, or EILM PEEN. 
The British Hymenophyllums, or Film Ferns, are small 
moss-like plants, with pellucid fronds, distinguished, along 
with Trichomanes, by having their fructification at the 
edges of the fronds ; and known from that genus by 
having the involucres, which surround the clusters of 
spore-cases, two-valved instead of urn-shaped or entire. 
So far as our native species go, these distinctions serve ; 
but they become puzzling in some exotic forms. They are 
the smallest of all our native Ferns, and, being somewhat 
rare, or at least local in their distribution, they have 
always been regarded with much interest. Two native 
species are recognized, much like each other in general 
aspect, and distinguished by one or two rather minute 
technicalities, which, however, are sufficiently obvious to 
those who have learned how to look for them. 
The name Hymenophyllum is compounded from the 
two Greek words hymen and phyllon^ which mean a mem- 
brane, and a leaf ; and is applied to those plants with 
