| 1 | Limestone plants; pinnae not over 4.5 cm long; no elongate glands on stalks of sporangia (sect. Leptochristella). |
| 1' | Plants not confined to limestone; pinnae commonly 10 cm or more long; elongate glands present on stalks of sporangia (sect. Christella). |
| 2 | Indusia distinctly present. |
| 2' | Indusia lacking or not clearly distinguishable. |
| 3 | Pinnae to 27 pairs, 5-7 lower ones gradually reduced |
| 3' | Pinnae c. 6 pairs, lowest only slightly reduced |
| 4 | Pinnae lobed almost to costae; veins all free |
| 4' | Pinnae lobed less deeply; veins near bases of pinnae anastomosing |
| 5 | Veins all free. |
| 5' | At least the basal veins anastomosing. |
| 6 | Red ellipsoid glands present on lower surface of pinnae |
| 6' | Such glands lacking |
| 7 | At least 3 pairs of lower pinnae gradually reduced. |
| 7' | At most 3 pairs of lower pinnae gradually reduced. |
| 8 | Sinus-membrane prominent on lower surface, 3 pairs of veins ending beside it; reduced pinnae hardly auricled |
| 8' | Sinus-membrane not prominent, at most 1.5 pairs of veins ending beside it; reduced pinnae clearly auricled. |
| 9 | Caudex strictly erect. |
| 9' | Caudex short-creeping, or at most suberect distally. |
| 10 | Reduced pinnae c. 8 pairs; hairs on lower surface of costae c. 0.1 mm long. |
| 10' | Reduced pinnae 3-4 pairs; some hairs on lower surface of costae 0.3 mm or more long. |
| 11 | Pinnae lobed less than half-way to costa; hairs on lower surface of costae all less than 0.5 mm long |
| 11' | Pinnae lobed more than half-way to costa; hairs on lower surface of costae mostly more than 0.5 mm long |
| 12 | Pinnae lobed at least half-way to costa; only 1 pair of veins truly anastomosing; some hairs 0.3 mm long present on lower surface of costae |
| 12' | Pinnae lobed ¼-⅓ towards costa; at least 1.5 pairs of veins truly anastomosing; all hairs on lower surface of costae c. 0.1 mm long |
| 13 | Hairs on lower surface of costae all very short. |
| 13' | Hairs on lower surface of costae always conspicuous, to 1 mm long. |
| 14 | Pinnae lobed c. ¼; minute capitate hairs present in addition to acicular hairs on lower surface of pinnae |
| 14' | Pinnae lobed c. 2/5; no capitate hairs on lower surface |
| 15 | Caudex long-creeping. |
| 15' | Caudex short-creeping or erect. |
| 16 | Pinnae lobed more than ½; apices of pinna-lobes not apiculate. |
| 16' | Pinnae lobed less than ½; apices of pinna-lobes apiculate |
| 17 | Basal 1-1.5 pairs of veins anastomosing, 1 pair passing to sinus-membrane |
| 17' | Basal 1 pair of veins anastomosing, at most the next acroscopic vein meeting the sinus-membrane |
| 18 | Pinnae lobed much more than ⅓; some hairs more than 0.5 mm long present on lower surface. |
| 18' | Pinnae lobed little more than ½; hairs on lower surface all less than 0.5 mm long |
| 19 | Caudex erect; 2 or more pairs of lower pinnae reduced |
| 19' | Caudex not erect; basal pinnae not reduced |
