622 FILICALES 
the lamina. These are all relatively primitive characters, and unusual in 
Ferns showing a mixed sorus: they direct the line of comparison down- 
wards to Matonia and Gleichenia. The former has a vascular system of 
the same type as Diéféeris, but it has run into greater complications, 
with its concentric solenosteles. Both genera, however, are considerably 
in advance of the most complex Gleichenias. Yet all these Ferns appear 
to conform in their various degrees of elaboration to the same vascular 
type. 
There is, however, no exact parallelism in the soral and vascular 
characters. Gleichenia is the most primitive in both respects; while 
Matonia is the most advanced of all in vascular structure, its sorus is still 
that of the Simplices, though it has only a small spore-output per sporan- 
gium, and a protective indusium is present; but as this is apparently 
absent in Zaccopteris, it has probably been in Aafonia a special generic 
feature. Dypieris, with its vascular system taking a middle place, has the 
most advanced soral condition, as shown by their distribution on the leaf, 
by the flat receptacle, and by the mixed aggregation of the sporangia in 
D. conjugata. But still it proclaims its origin by the absence of indusium, 
the oblique annulus, and the imperfectly differentiated stomium. The sum 
of characters justifies the conclusion that in Diépteris we see a genus of 
origin from a stock included in the Simplices, in which at least one species 
has passed, apparently without the intermediate state of a basipetal sorus, 
directly to the condition of the Mixtae. There is, moreover, good reason 
for holding that this phyletic line has proceeded quite independently of the 
other progressions to a mixed sorus which have been traced elsewhere. 
Finally, the palaeophytological data harmonise with this conclusion ; 
for representatives of the Dipteridinae figured largely in the Mesozoic 
Flora, as far back as the Rhaetic, with sori agreeing in form and distribution 
with these of Dzpteris; but the annulus is described as probably complete. 
This point may be considered doubtful; but if it were confirmed it would 
fall in readily with the phyletic position suggested for the Dipteridinae. 
The conclusion of Seward seems fully justified that AZatonia and Dipteris 
are linked together as remnants from a bygone age. They have advanced 
independently, the one to higher vascular complexity, the other to a distri- 
bution and construction of the sori characteristic rather of the more recent 
Ferns than of its own progenitors. 
It has now been seen that the condition of sorus characteristic of the 
Mixtae is absent from the Ferns which Palaeophytology tells us were the 
most primitive, but that it is the prevailing feature in the Ferns of the 
present day. It has also been seen that steps leading from the more 
primitive condition of the Simplices and Gradatae to the mixed type of 
sorus exist in certain Ferns: and further, that there is a probability that this 
end has been achieved by progression along more than one phyletic line: 
1Seward, /c¢., p. 507. 
