
44 THE BRITISH FERNS. 
openings towards the centre. The Danwinew have their sori con- 
nate over the whole under surface of the fertile fronds, which then 
show long parallel lines of small round cavities, which are the 
openings of the concrete spore-cases. 
The Polypodiacee offer so much variety of structure in their spore- 
cases that i& becomes necessary to range the genera under eight 
tribes or divisions, distinguished chiefly by peculiarities in the form 
of the spore-cases, in their number and position, or in the structure 
and development of the annulus or ring, which latter presents some 
curious differences. These tribes are as follows :一 
(1) Polypodinee, the most extensive of all, in which the spore- 
cases are almost equally gibbous or convex on both sides, with a 
vertical and nearly complete ring, and bursting transversely at 
a part on the anterior side, called the stoma, where the strie of the 
ring become dilated into elongate parallel cells. 
(2) Cyatheinec, in which the spore-cases are sessile or nearly so, 
and oblique-laterally compressed, the nearly complete ring being, in 
consequence, more or less obliquely vertical, that is, vertical below, 
curving laterally towards the top, bursting transversely, and seated 
on an elevated receptacle; they approach very near the Polypodinew 
through some species of Alsophila, in which the characteristic obli- 
quity of the ring is little apparent. 
(3) Matoninee, consisting of a single species only, in which the 
ring is broad, sub-oblique, and nearly complete, the spore-cases 
sessile, bursting horizontally, not vertically, the sori dorsal and 
oligocarpous, covered by umbonato-hemispherical indusia, which are 
peltate, that is, affixed by a central stalk; they may be compared 
to an inverted cup. 
(4) Gleicheninee, in which the ring is complete and transverse, 
either truly or obliquely horizontal, the spore-cases being globose- 
pyriform, forming oligocarpous sori, i. e., sori consisting of but few 
spore-cases (2-4 to 10-12), situated at the back of the frond, sessile - 

