Key to the species of Nervilia occurring in Africa
All characters used refer to unpollinated flowers in full anthesis (A) and to fully developed and mature leaves (B). Variable species are keyed out in more than one place. The variability of some of the species makes it necessary, in extreme cases, to know both flowering scape and leaf for a certain determination. However, most "single-aspect" specimens (leaf only or flowers only) are easily determinable.
Unfortunately, "geographical characters" have had to be used to separate the leaves of Nervilia pectinata and Nervilia subintegra. Apart from a difference in mean size they are indistinguishable.
Key to vegetative specimens of the genus Nervilia:

1 Leaf pubescent 2
  Leaf glabrous 7
2 Leaf distinctly polygonal or star-shaped in outline, thin, held a few cm above ground 3
  Leaf reniform to orbicular, often rather thick, mostly prostrate 4
3 From Malawi, Mozambique, Zambia, Zimbabwe or southern Tanzania Nervilia pectinata
  From Northern Hemisphere or lower Zaire Nervilia subintegra
4 A ± wide zone near main veins glabrous, leaf width 4-6 cm, Madagascar only Nervilia affinis
  Hairs also on or near main veins 5
5 Leaf small, < 3 cm across, purple below, reticulated with silver above (silvery pattern may be absent) Nervilia stolziana
  Leaf larger, 4.5-20 cm wide, green on both sides 6
6 Leaf 4.5-14 cm wide Nervilia crociformis
  Leaf larger, 18-22 cm across, Madagascar only Nervilia leguminosarum
7 Leaf base attenuate to truncate, leaf blade erect 8
  Leaf base cordate, leaf blade prostrate or raised 10
8 Leaf olive green with silvery veins above, purple below Nervilia ballii
  Leaf green on both sides 9
9 Leaf blade > 4 cm wide Nervilia shirensis
  Leaf blade < 4 cm wide Nervilia kotschyi vat. purpurata
10 Petiole very long, >10 cm; leaf held well above ground Nervilia bicarinata
  Petiole < 6 cm; leaf often more or less prostrate 11
11 Leaf pleated with ragged keels running along the ridges of the pleats Nervilia kotschyi var. kotschyi
  Leaf without such keels 12
12 Leaf large, > 9 cm wide, very pleated Nervilia renschiana
  Leaf smaller, < 9 cm wide, not pleated 13
13 Leaf very small, not more than 2 cm wide 14
  Leaf larger 15
14 Leaf with silvery rays (silvery pattern may be absent), ± orbicular, purple below Nervilia petraea
  Leaf without silvery pattern, broadly cordate, green below, Madagascar only Nervilia lilacea
15 Leaf above marbled or tesselate with silver or light green 16
  Leaf above uniformly green 17
16 Leaf very conspicuously tesselated silver and dark green, reniform, shortly apiculate or with rounded apex Nervilia adolphi var. seposita
  Leaf marbled in light and dark green, cordate, with prominent, triangular apical part Nervilia fuerstenbergiana
17 Leaf <5 cm wide 18
  Leaf > 5 cm wide 19
18 Leaf with triangular apex, not strongly prostrate, never dark purple below Nervilia lilacea
Leaf with more or less rounded apex, often strongly prostrate and dark purple below Nervilia adolphi var. adolphi
19 Leaf spade-shaped, slightly longer than wide, thick Nervilia kotschyi var. kotschyi
  Leaf reniform, cordate to orbicular, wider than long, ± thin 20
20 Leaf broadly heart-shaped with distinct apical point, not strongly prostrate Nervilia adolphi var. adolphi
  Leaf reniform to almost orbicular without distinct apical point, often strongly prostrate Nervilia crociformis

And species without a key:

Nervilia similis SW. Tanzania.
Nervilia hirsuta Madagascar.

World Checklist of Monocotyledons. The Board of Trustees of the Royal Botanic Gardens, Kew. Published on the Internet; accessed 06/11/2013