INTRODUCTION. 19 
of the ordinary development has occurred. Thus in the Scolo- 
pendrium vulgare, which normally has entire fronds and free veins, 
some of the monstrous varieties, in which the frond is narrowed 
and the margin broken up, have the veins so entirely disarranged, 
that they unite more or less frequently. We believe that all the 
instances which have been observed of inconstancy in the arrange- 
ment of the veins, may be referred either to an accidental confluence, 
or to some obvious disturbance of the normal development. 
The arrangement of the veins in Ferns may be brought under 
four heads, as follows :—(1), free, i. e., where the veins are continued 
without coming in contact with each other; (2), connivent, i. e., 
where all having an excurrent or outward tendency, they coalesce 
anglewise; (3), combined, i. e., where a longitudinal vein unites con- 
tinuously and transversely with other veins; and (4), reticulated, 1. e., 
where they form a complete network, being united in every direction. 
With the exception of Ceterach officinarum and Ophioglossum 
vulgatum, which have their veins reticulated, all British Ferns 
belong to the division with the veins free. There are one or two 
peculiarities still to mention. In some Ferns the venation is not 
developed beyond the first series from the costa—the veins, these 
remaining unbranched. In others, the costa itself is not present, but 
the whole venation consists of forked veins, venules and veinlets, 
branching in a flabellate or fan-shaped manner from the base 
of the pinna or pinnule; this occurs in Adiantum Capillus-veneris, 
and in Botrychium Lunaria, There is, moreover, to be. observed a 
difference in the mode in which the branching of the free veins is 
effected; some being pinnate or feather-branched, as may be well 
seen in the larger varieties of Lastrea Filiz-mas, while others, are 
forked or dichotomously-branched, as in Scolopendrium vulgare. 
It is, as we have elsewhere observed, the condition of the veins 
and the mode of their arrangement which has of late years been 
employed as an auxiliary character in distinguishing the genera 
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