190 HISTORY OF BRITISH FERNS. 



imaginable, it may be distinguished by this mark alone. 

 The fructification, too, is here totally nnlike that of all 

 others, except the Ilymenophyllums, 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 TricJiomanes 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 hack of the fronds. Trichomanes is known 

 from Ilymenophyllum by its urns, or involucres as they 

 are called, being entire, while those of Ilymenophyllum 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 Ilymenophyllum 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 Trichoma?ies itself has the same signification : 



