190 
HISTORY OF BRITISH FERNS. 
imaginable, it may be distinguished by this mark alone. 
The fructification, too, is here totally unlike that of all 
others, except the Hymenophyllums, from which, in the 
native species, it is easily distinguishable, although in some 
exotic kinds the differences almost vanish. The technical 
mark by which to distinguish Trichomanes and Hymeno¬ 
phyllum among the British Ferns, lies in the fact of their 
spore-cases being contained within deep urn-shaped pits or 
recesses at the margin; that is to say, in these two 
families the fructification is at the margin instead of being 
situated at the back of the fronds. Trichomanes is known 
from Hymenophyllum by its urns, or involucres as they 
are called, being entire, while those of Hymenophyllum are 
split lengthwise into two valves. In both, the spore-cases 
are clustered around hair-like receptacles, which are, in 
fact, the ends of the veins of the fronds projecting into the 
urns. In Hymenophyllum these hairs are always shorter 
than the urn, but in Trichomanes it is usual for them to 
project more or less, so that the fronds become somewhat 
bristly when very full of fructification; and hence has 
arisen the common name of Bristle Fern, which is applied 
to the group. 
The name Trichomanes itself has the same signification: 
