iv INTRODUCTION. 
by veins, producing, in British Ferns, one-celled cases con- 
taining the seed (spores). 
Mr. Sowerby having objected to our use of the classifica- 
tion we adopted in the First Edition of this work, and also 
to some of the magnified portions of the parts of fructifica- 
tion, we have been led to a closer consideration of these 
subjects, and we rejoice in being able to say that the resrlt 
of such consideration is a greater amount of correctness. 
The improved classification is as follows: 
POLYPODIACE^:. 
Fructification placed on the back of the frond, naked, 
having neither the usual covering nor covered by the margin 
of the frond. Eing vertical. Vernation coiled. 
CETERACH. Masses of fructification oblong, or nearly 
linear, straight, covering not apparent; mid-veins parallel 
or oblique, vein-branches uniting at their points. 
POLYPODTOM:. Masses nearly circular, scattered in spots, 
without covering. Edge of frond not bent back. 
GYMNOGKAMMA. Capsules seated on the forked veins of 
the fronds; covering none ; seeds triangular. 
Fructification placed on the back of the frond, and either 
furnished with a cover or having the margin of the frond 
turned back over it. Eing vertical. Vernation coiled. 
WOODSIA. Masses nearly circular, scattered \n dots; 
receptacle membranaceous, flat, somewhat plate-shaped, 
fringed with incurved hairs. 
PoiYSTiOHUsf. Masses circular, covering circular, fixed 
