192 kidston: the flora of the carboniferous period. 
fructification, but before referring to these more fully it is 
necessary very shortly to consider the fructification of existing 
ferns. 
Recent ferns are divided into two great classes — the Iso- 
sporous Ferns, or those with one kind of spore, and the 
Heterosporous Ferns, or those with two kinds of spores — 
macrospores and microspores. With the latter class, however, 
we have nothing to do at present. 
Returning to the Isosporous Ferns, these again form two 
great sections. First, those whose sporangia are provided with 
a prominent ring of cells, called an Annulus (Plate XXVIII., 
fig. 2), and those whose sporangia are destitute of this structure. 
The first section contains the great majority of recent ferns, 
of which the common Polypody and Male fern may be men- 
tioned as well-known representatives. The second group con- 
tains the Marattiacece, which comprises few genera and a small 
number of species, all of which are natives of more or less 
tropical areas. 
In Carboniferous times both these groups are represented, 
though the exannulate ferns seem to have outnumbered those 
with annulate sporangia. 
Let us now return to Sphenopteris. Many of the species 
originally included in that genus have in recent years been 
found showing their fructification, and for these new genera 
have been created. Among British Sphenopteroid forms a few 
are known to possess annulate sporangia, and of such are 
Corynepteris Baily and Oligocarpia Goppert. In the former the 
sporangia are placed in groups of five or six, united at the base 
around a common centre, and collectively form a globular mass 
or sorus ; in the latter they form little circular heaps composed 
of a number of independent sporangia. Isolated annulate 
sporangia are frequent in the Yorkshire and Lancashire " Coal 
Balls," and also occur in the material from Petty cur, Fife, which 
is situated in the lowest division of the Carboniferous Formation 
(Calciferous Sandstone Series). 
