GENERA OF FERNS. 379 
an indusiate genus of Gleicheniacee, rather than to retain it 
as an anomalous genus in Aspidiee. 
Gleicheniacee are readily discriminated from all other ferns 
by the almost constant dichotomous character of their fronds, 
only two species being known to have linear pinnate or pin- 
natifid fronds. The ultimate pinne or branches are usually 
deeply pinnatifid, or the lacinize are distant and sessile, the 
pinna being then pinnate, in every respect not unlike such 
ferns as Polypodium pectinatum, &c. About 30 species have 
been described; but their very great similarity and wide geo- 
graphical range render it very difficult to determine what 
are truly distinct species. From examination, I am induced 
to think that there may be about 20, and that under local 
influences they exhibit a difference in aspect, as in being 
more or less glaucous, smooth or tomentose, &c. which has 
been the cause of more species being described than really 
exist. The whole of the species agree with the genus Glei- 
chenia as characterised by Mr. Brown, but this admits of 
being separated into at least two natural groups or genera, 
the first containing the original species of Gleichenia, and the 
second consisting of those species characterised by Willde- 
now under his genus Mertensia. A solitary species constitutes 
the genus Platyzoma of R. Br., which differs from true 
Gleichenia more by its simple (not forked) fronds, than in any 
very evident and distinct character of the fructification. 
126. Puaryzoma, R. Br. 
Veins simple, obscure, free. Sporangia terminal, 2-4, 
forming round sori; margin of the pinne revolute, indusii- 
form, conniving, constituting a concave farinose cyst, that 
includes the sori. 
Rhizoma creeping, densely paleaceous. Stipes simple. Sterile 
fronds linear-filiform, 2-3 inches in length: fertile linear, 
pinnate, 3 to 12 inches long; pinne sessile numerous, rs of an 
inch in length, orbicular, revolute and plicate. 
Species. P. microphyllum, A. Br. 
Illust, Hook. gen. fil. t. 41. C. 
