The margin is continuously recurved to form a rather broad involucre, and the very edge is somewhat thinner and whiter. The veins are pinnately arranged on both sides of the midvein, and fork about twice before reaching the margin. The upper part of the veinlets is covered with sporangia, which as they ripen push out from beneath the involucre. The spores are obscurely tetrahedral and trivittate, as in the other species of the genus. This fern very often grows in company with _Camptosorus rhizophyllus_, and its root-stock is often hidden beneath mosses of the genus _Anomodon_: it takes kindly to cultivation, especially if it be planted in the crevices of calcareous rock-work. It may occur on other than calcareous rock, but I have never seen it on either granite, sandstone or basalt. Names for varieties of this species have been proposed by Pursh, and by Fournier, but the characters assigned do not seem sufficiently distinctive. PELLÆA GRACILIS, Hooker. Slender Cliff-Brake. Pellæa gracilis:—Root-stock slender, creeping, cord-like, scantily furnished with little ovate appressed scales; stalks scattered, slender, a span long or less, brownish-stramineous, somewhat shining, darker and slightly chaffy at the base; fronds two to four inches long, thin and tender, smooth, ovate or ovate-oblong, pinnate; pinnæ few, the lower two to four pairs once or twice pinnatifid, the uppermost simple; segments of the sterile fronds adnate-decurrent, roundish-obovate, crenately lobed and toothed; those of the taller fertile fronds lanceolate or linear-oblong, and more distinct, entire or auricled, terminal ones longest; veins rather distant, mostly once forked; involucre broad and continuous, delicately membranaceous. _Pellæa gracilis_, Hooker, Sp. Fil., ii., p. 138, t. cxxxiii, B.—Eaton, in Gray’s Manual, ed. v., p. 659; Ferns of the South-West, p. 319.—Hooker & Baker, Syn. Fil., p. 145.—Porter & Coulter, Syn. Fl. Colorado, p. 153. _Pteris gracilis_, Michaux, Fl. Bor.-Am., ii., p. 262.—Swartz, Syn. Fil., p. 99.—Willdenow, Sp. Pl., v., p. 376.—Pursh, Fl. Am. Sept., ii., p. 668.—Hooker, Fl. Bor.-Am., ii., p. 264. _Allosorus gracilis_, Presl, Tent. Pterid., p. 153.—Torrey, Fl. New York, ii., p. 486.—Gray, Manual, ed. i., p. 624; ed. ii., p. 591, t. ix.—Parry, in Owen’s Geol. Surv. of Wisconsin, etc., p. 621.—Mettenius, Fil. Hort. Lips., p. 44. _Cheilanthes gracilis_, Kaulfuss, Enum. Fil., p. 209. _Pteris Stelleri_, Gmelin, “Nov. Com. Petrop., xii., p. 519, t. 12, f. 1.” _Allosorus Stelleri_, Ruprecht, Distr. Crypt. Vasc. in Imp. Ross., p. 47.—Ledebour, Fl. Ross., iv., p. 526.—Moore, Ind. Fil., p. 46.—Lawson, in Canad. Naturalist, i., p. 272. _Allosorus minutus & Pteris minuta_, Turczaninow, _fide_ Moore.