five feet long (stipites very paleaceous), bipinnate, primary pinnz 
a span to a foot long (their rachis hispido-squamose), oblong-lan- 
ceolate, gradually acuminate, dark bright-green above, paler be- 
neath. Pinnules'close, compact, three-quarters to one inch long, 
coriaceo-membranaceous, elliptical, irregularly rhomboid, on short 
petioles articulated on the rachis, superior base truncated, the 
apex obliquely truncate, obtuse, rarely acute, subsinuato-serrate ; 
costa veniform, dividmg the pinnule into two very unequal 
halves; veinlets once or twice forked, clavate at the apex, a su- 
perior veinlet bearing the elliptical sorus. Involucre elliptical, 
attached by its centre to a large receptacle, and having a deep, 
narrow, grooved line at the sinus, not extending to the apex, 
giving an oblong horse-shoe-like shape to the involucre, which, 
however, is free all round at the margin, and this reflexed in age. 
The sori are mostly confined to the superior sides of the pinne, 
rarely seen on the lower and smaller half. 
The habit of this Fern is quite peculiar, and so is its fructifi- 
cation. The involucre is incorrectly described by the author of 
the genus as “‘geminate” (hence its name). It is as much a 
single or solitary involucre as any among the group of Ferns, 
originating at the apex of a veinlet, of an oval or elliptical form, 
with a small sinus at the base, and its point of attachment, for a 
considerable length and breadth, the centre of which is indicated 
by a depressed line or furrow, which seems almost to divide it 
into two halves, thus giving the appearance of a narrow hippo- 
crepiform (horse-shoe-shaped) involucre. This large point of at- 
tachment causes the involucre to be persistent. Yet the gene- 
ral form and structure is not far removed from that of Nephro- 
dium, and especially to that Nephrodium which has been called 
AMesochlena. See our figure and description of Vephrodium (Me- 
sochlena, Br.) Javanicum in our ‘ Filices Exotica,’ t. 72. 
Tt is quite certain that all the specimens discovered in Ame- 
rica, Asia, Africa, and the Pacifie islands, belong to one and the 
same species, nor do I find any strongly marked varieties. The 
pinnules indeed vary on different parts of one and the same 
plant, as is the case with many species of Adiantum, with which 
the form of the pinnules has no small resemblance. 
Pate 17. Fig. 1. Very much reduced figure of Didymochlena lunulata, 
Desv. 2. Fertile pinnee,—znat. size. 3. Fertile pinnule. 4, Young sorus. 
5. Old sorus :—more or leas magnified. 
