194 HISTORY OF BRITISH FERNS. 



Genus XVI. HYMENOPHYLLUM, or FILM FERN. 



The British HymenophiUums, 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 



