CLASSIFICATION 
2 7 
formed one means of establishing a basis of classification. 
But the chief mode of distinction or association has been 
suggested by the presence or by the absence of the scaly 
covering of the seed clusters, which, as we have seen, is called 
the indusium, as well as by the form of the latter, and the 
particular manner in which it maybe disrupted when the time 
arrives for the ripening and setting free of the spores. 
If we take those inhabitants of the Fern world which 
are to be found in Great Britain, Ireland, and the Channel 
Islands, we shall find that the whole of them may, under 
the system of classification based on the form and 
arrangement of the spore cases, be included under three 
principal groups comprehending lesser groups, which, in 
their turn, comprise genera, containing a varying number of 
species. The primary groups, which, as we have seen, are 
three in number, are called — 1. Polypodiaee x, a group con- 
taining ten lesser groups, including sixteen genera and 
forty-one species, and comprising Ferns whose fronds are, on 
starting from the crown, found to be rolled up in a circinate 
or scroll-like manner, and having their spore cases surrounded 
by an elastic ring, which, when it bursts, does so by a 
transverse fracture. 2. Osmundacea ?, comprising in Britain 
only one genus, of which there is but one species. The group, 
however, comprised under Osmundacese includes Ferns which, 
although their young fronds are rolled up like those included 
under Polypodiaceae, have no elastic ring round their spore 
cases, the latter, consisting of two valves, bursting in a 
vertical manner. 3. Ophioglossace te, comprising two genera 
and three species in Britain, and including Ferns whose 
fronds are not rolled in in a circinate or scroll-like manner, 
but are folded up straight, and whose spore cases are — like 
the Ferns comprised under Osmundacea? — deprived of an 
elastic ring, and two-valved. 
