Introduction. 
xi 
point for a primary division is found in the existence or absence of an indusium to the sori. 
This is the first thing to be looked for, and from what has been said above, in our description of 
the indusium, it will be seen that the necessary examination must be careful, or the indusium 
EREMOBRYOID RHIZOME OF POLYI'ODIUM 
VULGARE {half natural size), SHOWING 
SCARS OF FALLEN FRONDS. 
DESMOBRYOID RHIZOME OF PTER1S AQUILINA ( half natural size). 
(After Sachs.) 
really present will be overlooked. Taking the whole 
world, there are recognised eleven distinct tribes of Poly- 
podiaccce — eight with an indusium and three without it. 
Of these two (in each division, one) have no European 
representative. The differences are briefly given in the table 
of classification below, and will be seen to depend chiefly 
on the nature and form of the indusium (where present), and the form and position of the 
sori. Under each tribe we have given the genera (nineteen in all) of the European Fern-flora, 
with the distinguishing character of each, so that it is hoped that the reader will be able to 
ascertain the genus of an unknown species without much difficulty. For the description of 
the species themselves he must of course refer to the body of the work itself, to which this 
sketch is only intended as an introduction. The sub-order Hymenophyllacece , which is here 
placed at the head of the list, follows the Dicksoniece in the body of the book. The classifi- 
cation involves the use of a number of technical terms which at first sight appear somewhat 
alarming ; but an explanation of all of them will be found in the preceding pages. 
CLASSIFICATION OF THE GENERA OF THE FERNS OF EUROPE. 
Sub-Order I, HYMENOPHYLLACECE. — Sporangia sessile, rounded but compressed, surrounded by 
a complete transverse ring, splitting vertically, and seated on a long, stalk-like receptacle, 
which is terminal or marginal, continued from the end of a vein. Indusium inferior, half- 
cup-shaped. Ferns with membranous filmy fronds, and a filiform, long, creeping caudex. 
Genus I. Trichomanes. — Sporangia occupying the base of the very long receptacle 
which is exserted beyond the truncate indusium. 
Genus 2. Hymenophyllum. — Sporangia occupying the whole (or nearly so) of the 
shorter receptacle which is contained within the two-lipped indusium. 
[Sub-Order Glcicheniacece. — Sporangia sessile, surrounded by a complete transverse ring, 
splitting vertically, very few (from 2 to io) combining to form dorsal sori, which have a 
short receptacle. No indusium. Ferns with a creeping caudex and rigid dichotomously- 
branched fronds. No European genus.] 
