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1.  Sori on the edge of a pinnule terminating a vein; sporangia at the base of a long, bristle-like receptacle surrounded by a cup-shaped indusium.

Filmy Fern. Trichomanes.

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2.  Indusium formed by the reflexed margin of the pinnules.

(1) Sporangia on a continuous line; fronds large, ternate; indusium narrow.  Bracken.  Brake. Pteris.

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(2) Sporangia in oblong sori under a reflexed tooth of a pinnule; indusium broad; rachis dark and shining.  Maidenhair. Adiantum.

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(3) Sori in roundish or elongated masses.

Indusium broad, nearly continuous, fronds mostly smooth, somewhat leathery, pinnate.  Rock species.  Cliff brakes. Pellaea.

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Indusium narrow, seldom continuous, formed by the margin of separate lobes or of the whole pinnules; often inconspicuous, fronds usually hairy.  Lip Ferns. Cheilanthes.

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Indusium of the reflexed edges, at first reaching to the midrib, or nearly so; later opening out nearly flat; fruiting pinnules pod-like; sterile fronds broad.  Rock brakes. Cryptogramma.

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3.  Indusium never formed of the margin of the frond.  Sori various.

(1) Fruit-dots oblong, parallel with the midrib, somewhat sunken in the tissues of the frond.  Water-loving species.  Chain Ferns. Woodwardia.

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(2) Fruit-dots and indusium roundish.

Indusium shield-shaped, fixed by the center.  Evergreen glossy ferns in rocky woods.  Shield Ferns. Polystichum.

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Indusium cordate, fixed by the sinus.  Wood Ferns. Aspidium.

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Indusium hood-shaped, fixed centrally behind the sorus and arching over it, soon withering, often illusive.  Fronds two to three pinnate, very graceful.  Moisture-loving species.  Bladder Ferns. Cystopteris.

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Indusium star-shaped, of a few irregular segments fixed beneath the sorus, often obscure.  Mostly small, rock-loving plants, usually rather chaffy, at least at the base, and growing in tufts. Woodsia.

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Indusium cup-shaped, fixed beneath the sorus, supported by the tooth of a leaf; sporangia borne in an elevated, globular receptacle open at the top.  Fronds finely cut.  Hayscented Fern. Dennstaedtia.

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(3) Fruit-dots and indusium linear. (But see Athyrium.)

Very long, nearly at right angles to the midrib, double; blade thick oblong-lanceolate, entire; heart-shaped at the base.  Hart’s Tongue. Scolopendrium.

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Shorter and irregularly scattered on the under side of the frond, some parallel to the midrib, others oblique to it, and often in pairs or joined at the ends; blade tapering to a slender tip.  Walking Fern. Camptosorus.