Key to the genus Benthamia A.Rich.
Translated from French to English as best I could

1a Cylindrical spur, having at least 2 mm. 5 long  2
1b Shorter, rounded, scrotiform or bagged spur 8
2a Leaves reduced to sheaths or not yet developed at flowering  3
2b Leaves developed at flowering (sometimes only one, radical)  4
3a Small plant, aphylla; glabrous tubers; bifid spur at the top (massif d’Andringitra, vers 2.300 m)  Benthamia exilis 
3b More robust plant, surrounded at the base with a rosette of leaves (8-10), not developed at flowering; hairy
tubers; whole spur (S. de Fianarantsoa, de 1.000 à 1.400 m) 
Benthamia praecox
4a Clearly three-lobed lip 5
4b Whole lip, sometimes dilated near the base  6
5a A single leaf (radical), narrow; spur hanging, as long as the ovary; unilateral flowers of a sulfur yellow
(Ankaratra et Andringitra, de 1.800 à 2.000 m.) 
Benthamia glaberrima
5b 5-6 stem leaves, wide (up to 75 mm); shorter spur (4 mm) than the ovary; non-unilateral flowers (Mt Ibity,
vers 2.000 m.) 
Benthamia bathieana
6a A single developed leaf; plant 15-30 cm; tubers 2;2 mm spur 5; sepals 3 mm, uninervated; petals with 2 ribs;
staminodes 2 times shorter than anther (Andringitra massif, 2.600 m). 
Benthamia monophylla
6b Two leaves developed; taller plants (45 à 70 cm.) 7
7a Spur 3 mm 5; lip dilated strongly below the middle; anther retusive, more than 1 mm high; very rounded
staminodes (Ankaratra, 2.000 m.) 
Benthamia perularioides 
7b 5 mm spur; diamond-shaped lip; shorter apiculate anther (Mt Ibity et Andringitra, 2.000 m.)  Benthamia rostrata 
8a Whole or obscurely toothed or three-lobed lip, but with lateral lobes not reaching half of the median lobe 9
8b Three-lobed lip, lateral lobes at least 2/3 of the middle lobe or only trisinuated, but in this case
(B. hermimoides  var. angustifolia) with 3 rounded and equal teeth 
9a Epiphytic plants; leaves generally broad (at most 4 times longer than broad); ear with flowers, unilateral or not;
linear and sickle-shaped staminodes, equal to or above the anther; flowers often 5 mm long (without ovary)
9b Terrestrial orchid; leaves generally narrow and more than 4 times longer than wide; spike with scattered
flowers; broad staminodes, shorter than anther or at most equal; often smaller flowers
10a Labelle hollow in obtuse hoof; clearly apiculate anther (Vallée d’Ambolo, E.)  Benthamia catatiana 
10b Labelle concave at the base, but not in the shape of a very hollow and obtuse hoof; anther retusive or
obscurely apiculate 
11a Lower leaf broadly obovate-cuspid, with obtuse cusp; very long staminodes (1 mm. 5), very narrow; laterally
toothed lip; median lobe of the whole rostella; spike cylindrical, not unilateral (Mananara, N. E.)
Benthamia cuspidata 
11b Lower leaf attenuated-acute or sometimes obtuse, but not cuspid-obtuse; shorter staminodes; whole lip;
median lobe of the tridenticulate rostella; one-sided flowers 
Benthamia nigrescens 
  Subspecies of Benthamia nigrescens Schltr.   
A1 More than 3 sheets  B
A2 Up to three sheets  C
B1 Anther shortly apiculate, not surpassed by staminodes; flowers small (3 mm. 1, without ovary); slightly
unilateral inflorescence (Réunion)
subsp. borbonica  
B2 Antere retuse, somewhat overwhelmed by staminodes; larger flowers (4 mm. 5, without ovary); infl. markedly
unilateral; median lobe of whole rostella (Réunion)
subsp. secundiflora  
B3 Antere retuse, exceeded by staminodes by a third of its length; even larger flowers (5 mm. 5, without
ovary); infl. not very clearly unilateral; median lobe of the tridenticulate rostella at the apex (Ankaratra) 
subsp. nigrescens 
C1 Staminodes longer than anther retuses; obtuse leaves (E. et Tsaratanana)  subsp. humblotiana
C2 Staminodes shorter than the apiculate anther; acute leaves (Ankaizina) subsp. decaryana 
12a Entirely whole lip, dug in hoof  13
12b Labell more or less toothed or lobed laterally 14
13a Flowering stem leafy at the base, bearing 2 oval-cuspid leaves; bracts 2 times longer than flowers; oval lip in
its outline (Imerina) 
Benthamia misera
13b Naked flowering stem, the single leaf carried with 2 sheaths by a lateral bud; shorter bracts (Andratamarina,
dans le N. E.) 
Benthamia calceolata 
14a Trineve medial sepal; narrow, erect leaves; wide and short staminodes (Andringitra, 2.500 m.).......  Benthamia macra  
14b Unineral median sepal  15
15a Lower sheaths of deep black; apiculate-acute anther; broad staminodes, shorter than anther (Ikongo,
à l’E. d’Ivohibe) 
Benthamia nigrovaginata
15b Green lower sheaths 16
16a Staminodes much shorter than anther; flowers small (at most 3 mm., without ovary) 17
16b Staminodes equal to anther or longer  19
17a Oval-lanceolate leaves, less than three times longer than wide (Mt Tsaratanana) Benthamia herminioides subsp. intermedia
17b Narrow leaves, 7 to 10 times longer than wide  18
18a Compressed spur, longer and almost bidentate at the top (Fort-Dauphin) Benthamia dauphinensis 
18b Cylindrical spur, shorter, whole (eastern side of the island and Mascareignes  Benthamia africana 
19a Lateral lobes of the labellum subnuls; 2.5-3 mm sepals. long; narrow leaves, 12 mm. wide at most (bassin
supérieur du Mangoro) 
Benthamia procera 
19b Lateral lobes of the lip equal to half of the median lobe; 3 mm sepals. 5; leaves up to 35 mm. wide (massif
de Manongarivo, 1.200 m 
Benthamia verecunda 
20a Staminodes equal to anther or longer, often linear and curved in false shape 21
20b Broad staminodes, shorter than anther 23
21a Epiphytic plants; wide and short leaves (42 x 14 to 120 x 45 mm.); uninervate sepals; white flowers on
one side 
Benthamia nivea  
  Subspecies of Benthamia nivea Schltr.  
A1 Flowers (without ovary) of 8-10 mm; lip, 5-level (Mt Tsiafajavona, 2.000 m.)  subsp. nivea 
A2 Flowers of 5, 5-6 mm; 3-rib lip (Mt Tsaratanana, vers 2.000 m.) subsp. parviflora
21b Terrestrial orchid; narrow leaves; uninerve or trinerve median sepal; white or yellowish flowers, unilateral
or not 
22a Trineve medial sepal; flowers yellowish or greenish, small (less than 4 mm.), scattered; 5-6 sheets,
measuring  up to 160 x 20 mm; linear sickle-cell staminodes (E.?) 
Benthamia elata  
22b Unineral median sepal; flowers twice as large (8-9 mm., without ovary), white, unilateral spike; oblong
staminodes, fairly wide (Fort Dauphin) 
Benthamia madagascariensis
23a Robust plants (stem 6-10 mm. In diameter); broad (20-40 mm.) and thick leaves; dense spike; wide petals,
less than twice as long as wide
23b Smaller plants; thin and narrow leaves, sometimes nevertheless ovate-lanceolate but then small; loose
spike; narrower petals
24a Black lower sheaths; uninervate sepals and petals; obtuse anther; palace of the lip without hull  Benthamia humbertii 
24b Green sheaths; trinerval sepals and petals; apiculate anther; palace of the lip decorated with 3 hulls
Benthamia chlorantha
25a Unineral median sepal; 2-3 leaves, often ovate-lanceolate (at most 50 x 25 mm.); sparse spike  Benthamia herminioides 
  Subspecies of Benthamia herminioides Schltr.  
A1 Leaves towards the middle of the stem, lanceolate-linear; lateral lobes of the lip obliterated; obtuse
rostolle median lobe (Mt. Tsaratanana) 
subsp. angustifolia 
A2 Leaves at the bottom of the stem, ovate-lanceolate  B
B1 Median lobe of the subangulate-acute rostella; rounded side lobes of the lip (Mt. Ibity)  Benthamia herminioides
B2 Median lobe of obtuse rostella; narrow and acute lateral lobes of the lip (Taratanana)  subsp. arcuata  
B3 Median lobe of obtuse rostella; rounded side lobes of the lip (Tsaratanana), about 2 times shorter than
the median
subsp. intermedia
25b Trineve medial sepal; always narrow leaves 26
26a Black base sheaths; median sepal less than 3 mm. long; dense and long cylindrical spike
(Mt Tsiafajavona, 2.000 m.)
Benthamia melanopoda
26b Green base sheaths; median sepal 4 mm; interrupted spike, fewer flowers (Imerina et Betsileo) Cynorkis cinnabarina