| 1 | Costae not grooved on upper surface; veins not reaching margin. |
| 1' | Costae grooved on upper surface; free veins all reaching margin. |
| 2 | Fronds bipinnate. |
| 2' | Fronds simply pinnate. |
| 3 | Scales 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 |
| 4 | Stipe-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 |
| 5 | Sori indusiate |
| 5' | Sori exindusiate |
| 6 | Fronds 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. |
| 7 | Spores trilete; caudex short, erect; on rocks by streams |
| 7' | Spores normally monolete; caudex and habitat various. |
| 8 | Rhizome long-creeping in wet ground; broad flat scales present on lower surface of costae. |
| 8' | Not this combination of characters. |
| 9 | Veins free |
| 9' | Veins anastomosing. |
| 10 | Fronds not proliferous; sori indusiate |
| 10' | Fronds proliferous; sori exindusiate |
| 11 | Caudex 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. |
| 12 | Caudex massive, erect; scales narrow, rigid, brittle; sori close to costules, exindusiate or with very small indusia. |
| 12' | Not this combination of characters. |
| 13 | Sori elongate along veins, exindusiate; sporangia bearing slender straight setae |
| 13' | Sori round, or if elongate and exindusiate the sporangia bearing hooked hairs or none. |
| 14 | Veins free. |
| 14' | Veins anastomosing. |
| 15 | Basal 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. |
| 16 | Rhizome long-creeping; no glands nor hairs on sporangia. |
| 16' | Rhizome not long-creeping; sporangia in some cases with glands or hairs. |
| 17 | Rhizome slender; pinnae rarely over 6 cm long |
| 17' | Rhizome 5-7 mm diameter; pinnae 10 cm or more long |
| 18 | Bases of lower pinnae not greatly narrowed. |
| 18' | Bases of lower pinnae much narrowed |
| 19 | Pinnae rigid, commonly at least 10 x 1 cm |
| 19' | Pinnae thin, to 3 cm long, rarely 1 cm wide. |
| 20 | Sporangia bearing glands. |
| 20' | Sporangia lacking glands |
| 21 | Pinnae 20-30 pairs, no glands on their lower surface. |
| 21' | Pinnae c. 7 pairs; glands present |
| 22 | Spores coarsely tuberculate or ridged; sporangia lacking glands. |
| 22' | Spores with a thin wing and cross-wings or many small thin wings. |
| 23 | Basal pinnae much narrowed at their bases |
| 23' | Basal pinnae not much narrowed at their bases which are in most cases auricled on acroscopic side |
| 24 | Pinnae 20 pairs or more, 2-3 cm long |
| 24' | Pinnae much longer, or fewer. |
| 25 | Pinnae or simple fronds subentire; lower surface between veins often pustular |
| 25' | Pinnae deeply lobed; surface between veins not pustular. |
| 26 | Hooked hairs present on lower surface of frond |
| 26' | Hooked hairs lacking. |
| 27 | Rhizome slender, long-creeping; many lower pinnae gradually reduced; some septate acicular hairs on lower surface |
| 27' | Not this combination of characters. |
| 28 | Caudex 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. |
| 29 | Basal 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. |
| 30 | Veins 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 |
| 31 | Body 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 |
