1Costae not grooved on upper surface; veins not reaching margin.
1'Costae grooved on upper surface; free veins all reaching margin.
2Fronds bipinnate.
2'Fronds simply pinnate.
3Scales on lower surface of frond, if present, consisting of a row of cells with red cross-walls acicular hairs all unicellular
3'Scales on axes of frond thickened at base, or replaced by long slender septate acicular hairs
4Stipe-scales bearing superficial hairs; rachis-wing, if present, narrow and of even width.
4'Stipe-scales with marginal hairs only; pinnae connected by wings along the rachis forming semicircular lobes between pinnae
5Sori indusiate
5'Sori exindusiate
6Fronds simple, or pinnate with lower pinnae not decrescent (small basal pinnae inconstantly present in Amphineuron).
6'Lower pinnae gradually reduced, or an abrupt change to small pinnae at base of frond.
7Spores trilete; caudex short, erect; on rocks by streams
7'Spores normally monolete; caudex and habitat various.
8Rhizome long-creeping in wet ground; broad flat scales present on lower surface of costae.
8'Not this combination of characters.
9Veins free
9'Veins anastomosing.
10Fronds not proliferous; sori indusiate
10'Fronds proliferous; sori exindusiate
11Caudex erect; sporangia unstalked, lacking hairs and glands; some sessile resinous glands on lower surface in most species; plants of mountain ridges
11'Not this combination of characters.
12Caudex massive, erect; scales narrow, rigid, brittle; sori close to costules, exindusiate or with very small indusia.
12'Not this combination of characters.
13Sori elongate along veins, exindusiate; sporangia bearing slender straight setae
13'Sori round, or if elongate and exindusiate the sporangia bearing hooked hairs or none.
14Veins free.
14'Veins anastomosing.
15Basal basiscopic vein of each group arising from costa below the attachment of its costule; a red gland at the end of hairs on stalks of sporangia
15'Basal basiscopic vein of each group not always thus arising; hairs on sporangium-stalk otherwise.
16Rhizome long-creeping; no glands nor hairs on sporangia.
16'Rhizome not long-creeping; sporangia in some cases with glands or hairs.
17Rhizome slender; pinnae rarely over 6 cm long
17'Rhizome 5-7 mm diameter; pinnae 10 cm or more long
18Bases of lower pinnae not greatly narrowed.
18'Bases of lower pinnae much narrowed
19Pinnae rigid, commonly at least 10 x 1 cm
19'Pinnae thin, to 3 cm long, rarely 1 cm wide.
20Sporangia bearing glands.
20'Sporangia lacking glands
21Pinnae 20-30 pairs, no glands on their lower surface.
21'Pinnae c. 7 pairs; glands present
22Spores coarsely tuberculate or ridged; sporangia lacking glands.
22'Spores with a thin wing and cross-wings or many small thin wings.
23Basal pinnae much narrowed at their bases
23'Basal pinnae not much narrowed at their bases which are in most cases auricled on acroscopic side
24Pinnae 20 pairs or more, 2-3 cm long
24'Pinnae much longer, or fewer.
25Pinnae or simple fronds subentire; lower surface between veins often pustular
25'Pinnae deeply lobed; surface between veins not pustular.
26Hooked hairs present on lower surface of frond
26'Hooked hairs lacking.
27Rhizome slender, long-creeping; many lower pinnae gradually reduced; some septate acicular hairs on lower surface
27'Not this combination of characters.
28Caudex erect; pinnae to c. 5 cm long, lower ones gradually reduced; sporangia sessile, lacking glands or hairs; plants of high mountain ridges
28'Not this combination of characters.
29Basal scales on stipe in almost all cases broad and thin; lamina between veins on upper surface almost always smooth and glabrous, lower surface of costae never densely hairy; glandular hairs on lower surface, if present, minute and colourless.
29'Basal scales narrow; upper surface between veins often bearing hairs and/or glands, lower surface of costae usually with copious hairs; sessile spherical or elongate glands often present.
30Veins free; reduced basal pinnae consisting of an aerophore with a minute lamina; pinnae not pustular between veins on lower surface when dried
30'Veins anastomosing, or if free the reduced basal pinnae with a distinct green lamina; lower surface of pinnae between veins ± pustular when dried
31Body of sporangium lacking hairs or glands; an elongate unicellular glandularhair on sporangium-stalk, similar hairs sometimes also on lower surface of veins; spores coarsely tuberculate or ridged
31'Body of sporangium usually with sessile spherical glands or setae; hairs on sporangium-stalk consisting of several cells; spores with many small thin wings or a continuous wing with cross-wings; sessile spherical glands present on lower surface in many species