INTRODUCTION， 25 
The position of the receptacle on the vein is variable, although 
generally uniform in the same species. Sometimes it occurs at the 
apex of the vein, and it is then said to be terminal. Less fre- 
quently it is at the point where the vein leaves the costa, and it is 
then said to be basal. Sometimes it is placed at a point between 
the apex and the base, when it is medial. It is sometimes trans- 
versely continuous, combining, as it were, the adjacent veins cross- 
wise. Sometimes it occurs at a point where two or more veins 
unite, when it is called compital, but there are no examples of this 
structure among British Ferns. 
The spore-cases of Polypodiaceous Ferns, which group includes the 
greater number of known species, are small roundish or obovate, 
hollow laterally compressed one-celled bodies, nearly surrounded by 
an elastic belt or Ring, which ring is sometimes called the Annulus, 
and hence the Ferns of this group are sometimes named the Annu- 
late Ferns. In the Marattiaceous and Ophioglossaceous Ferns, the 
spore-cases are mere one-celled cavities without any trace of ring or 
annulus, and these plants are consequently called Exannulate Ferns. 
The exannulate Ferns, which comprise only the Marattiacee and 
the Ophioglossacew, have their spore-cases dorsal on normal fronds in 
the former group, but collected upon the sides or surface of con- 
tracted fronds in the latter. The only exannulate British Ferns 
are the Botrychium Lunaria and the Ophioglosswms. 
The ring of the spore-case takes a different direction in different 
groups of Ferns, but always forms a single line, partly or entirely 
encircling the spore-case, and always occupying a similar position 
in Ferns of the same group. The position and nature of the ring 
have, in consequence been taken as affording the best technical 
characters for the purpose of arranging Ferns into primary groups. 
Some of the annulate Ferns have the ring in a rudimentary condi- 
tion, but in the majority of British species belonging to the Poly- 
podinee, a division of the Polypodiacee, it is nearly complete, and 

