Description.—The root-stocks of this little fern are creeping, branched and often entangled, and chaffy with narrow lance-acuminate dark-fuscous scales. The cellular structure of these scales is similar to that of the scales of _A. ebeneum_, the cells being oblong-rectangular, and arranged in straight longitudinal rows. The stalks are from two to four inches long, and slightly chaffy when young: they are brown and shining at the base, but green higher up, except that a narrow line of brown is continued up the under side of the stalk nearly or quite to the base of the frond. A section made near the lower extremity of the stalk is nearly semicircular, and discloses two roundish fibro-vascular bundles side by side near the middle, and a minute thread of sclerenchyma, or hard dark tissue, on the inner side of each bundle. A section just below the frond shows the two fibro-vascular bundles united into one, and the angles of the stalk slightly extended, forming very narrow wing-like borders. The minute inner filaments of sclerenchyma are never continued far up the stalk, and are sometimes wanting altogether.
The frond is from three to six inches long, and usually half an inch to an inch broad at the base, from which the general outline tapers to a long and slender point, not so long as the prolongation of the walking-leaf, and very rarely, if ever, rooting at the apex.[4] The fronds are mostly erect, sub-coriaceous or firmly membranaceous, smooth above, but with a few minute setulose scales beneath, deeply pinnatifid in the lower and middle portion, and sinuately lobed above, the long terminal portion undulate on the margins. The midrib is broad and well defined: it is winged throughout its length; the wing narrow at the base of the frond, but constantly widening upwards.
The lobes are irregularly roundish-ovate, sinuate, crenate or slightly toothed; the lowest ones occasionally drawn out into an acuminate point an inch long. Most of the lobes are attached to the wing of the midrib by a broad base: the lower ones sometimes have a short stalk.
The veins are everywhere free: in the lower lobes, if these are acuminate, the veins are pinnately branched from a mid-vein; elsewhere they are forked or dichotomous. The sori are mostly single, though here and there one will be diplazioid,—most commonly the lowest one on the superior side of the lobe. The indusia are very delicate; and the free edge is directed toward the middle of the lobe, excepting the indusia of the sori nearest the midrib, and these open toward the midrib. The sori are usually very full of sporangia, and, when ripe, nearly cover the back of the frond: even the narrow acumination bears a sorus at each undulation of the margin. Spores ovoid-bean-shaped, with reticulating ridges and an irregular winged border.