_Allosorus minutus & Pteris minuta_, Turczaninow, _fide_ Moore.
Hab.—Crevices of damp and shaded calcareous rocks, especially in deep glens; Labrador, Butler, to British Columbia, and southward to Iowa, Parry, Wisconsin and Pennsylvania. Also in Colorado, near Breckinridge City, Brandegee. Siberia, Tibet and the Himalayas. It is found in Sunderland, Massachusetts; at Trenton Falls, Chittenango Falls, and other deep glens in Central New York; in Lycoming and Sullivan Counties, Pennsylvania, and in other similar places in Vermont, Michigan, etc., but is by no means a common plant.
Description:—This is the most delicate of all the _Pellæas_, and has fronds a good deal like those of _Cryptogramme acrostichoides_, but tenderer, and with sub-marginal fructification. The root-stock is very slender, scarcely more than half a line in thickness, and sometimes two or three inches long. It is so hidden in the crevices of the rocks that it is seldom secured by collectors. The scales are minute, appressed to the root-stock, and almost filmy in their delicacy.
The stalks are scattered along the root-stock, and are generally about five or six inches long, those of the fertile fronds longer, stouter and of a darker color than the others. They are smooth and somewhat polished, but lighter in color and far more tender in consistency than in most of our other species of this genus.
The fertile and the sterile fronds are unlike, though both are very delicately membranaceous, and pinnate with once or twice pinnatifid pinnæ. The rachis is not winged in its lower half, except in very small fronds, but above the middle it is narrowly winged, as are also its divisions. The lowest one or two pairs of pinnæ are twice pinnatifid in the largest specimens, but more commonly but once pinnatifid. In the sterile fronds the segments of the pinnæ are very plainly adnate to the secondary midrib, and are roundish or roundish-obovate in shape. They are from three to six lines long and about two-thirds as broad. Their margin is more or less lobed and crenately toothed. In the fertile fronds the segments are more distinct, longer and narrower, measuring often six to ten lines in length and one or two in width. The terminal pinna of the frond and the terminal segments of the pinnæ are considerably longer than the others. The veins are conspicuous, and distant, much more so than in our other species of _Pellæa_. They fork once about midway between the midvein and the margin, and sometimes, especially in fertile fronds, a second time just within the margin.