Stems creeping, not cordlike, internodes very short, less than 5 mm, heavily beset with old petiole bases, hairs absent; scales uniformly brown to ± clathrate, radial walls brown, luminae clear. Leaves monomorphic, clustered at stem apex, to 45 cm, nearly all bearing sori. Petiole usually dark at base, grading to straw-colored distally, shorter than blade, sparsely scaly at base. Blade ovate to narrowly ovate, 2-pinnate to 2-pinnate-pinnatifid, widest above base, apex short-attenuate; rachis and costae usually sparsely invested with unicellular, gland-tipped hairs, occasionally with misshapen bulblets; axils of pinnae with occasional multicellular, gland-tipped hairs. Pinnae typically perpendicular to rachis, not curving toward blade apex, margins serrate; proximal pinnae pinnate-pinnatifid to pinnatifid, ± equilateral, basiscopic pinnules not enlarged, basal basiscopic pinnules sessile to short-stalked, base truncate to obtuse; distal pinnae ovate to oblong. Veins directed into teeth and notches. Indusia cup-shaped, apex truncate, typically sparsely invested with unicellular, gland-tipped hairs. Spores spiny, usually 49--60 µm. 2 n = 252. Sporulating summer--fall. Cracks and ledges on cliffs, often on calcareous substrates; 0--1000 m; N.B., Nfld., N.S., Ont., Que.; Conn., Ill., Iowa, Mass., Mich., Minn., Pa., Vt., Wis. Cystopteris laurentiana is a sexual allohexaploid species with C . bulbifera as the diploid parent and C . fragilis as the tetraploid. Cystopteris laurentiana was previously thought to be common only in the Great Lakes region (R. F. Blasdell 1963); it is now known to occur frequently in the Driftless Area of the Midwest. Because C . laurentiana can be difficult to distinguish from C . fragilis , specimens with ovate leaves having unusually large spores and growing on moist cliffs should be checked carefully for occasional glandular hairs, the distinguishing feature of C . laurentiana . Sterile pentaploid hybrids between C . laurentiana and C . fragilis have been discovered where the two species are sympatric.