Benthamia A.Rich., Mém. Soc. Hist. Nat. Paris 4: 37 (1828).
Heterotypic Synonyms
Rolfeella Schltr., Repert. Spec. Nov. Regni Veg. Beih. 33: 18 (1924).
The genus Benthamia A. Rich. was first described by Achille Richard in 1828, the name being a tribute to George Bentham, the British botanist. It was created to accommodate the already existing Satyrium latifolium Thouars (today Benthamia chlorantha (K. Sprengel) Garay & Romero), and Satyrium spirale Thouars, which is henceforth Benthamia spiralis (Thouars) A. Rich.

The genus is characterized by the following features: terrestrial or rarely (two species) epiphytic herbs, with 2 to 7 ovoid tubers. Stem with only sheaths or 1-2 basal leaves or 2-5 cauline leaves. Inflorescence terminal, with a lax to dense spike, cylindrical, more or less secund (i.e. arranged on one side of the axis only), or secund. Flowers small (less than1 cm), white, whitish, yellow, yellowish, green, greenish or reddish orange. Lip entire or, more often, trilobed, with a short spur at the base, generally scrotiform, globular or saccate, but also cylindrical or clavate. Column with parallel anther locules. Viscidia flat, oval or elliptic. Staminodes well developed, subspathulate or falcate. Rostellum small, trilobed or tridentate. Stigmas always very short. Pollinia 2. The last comprehensive treatment of the genus with full description of species dates back to Perrier’s work in 1939.

A revision of the whole genus, along with genetic studies to verify its monophyletic origin, is much needed.
W. Indian Ocean
Key to the genus Benthamia
Translated from French to English as best I could

Species without a key:

Benthamia boiteaui Hervouet   Madagascar
Benthamia bosseri Hervouet   Madagascar 
Benthamia exilis var. exilis  SC. Madagascar 
Benthamia exilis var. tenuissima Schltr.   C. & SC. Madagascar 
Benthamia flavida Schltr.   N. & C. Madagascar 
Benthamia herminioides subsp. herminioides  NE. & C. Madagascar 
Benthamia longicalceata H.Perrier NE. Madagascar 
Benthamia majoriflora H.Perrier NE. Madagascar 
Benthamia perfecunda H.Perrier  N. Madagascar 

World Checklist of Selected Plant Families. The Board of Trustees of the Royal Botanic Gardens, Kew. Published on the Internet; accessed 03.04.2020
Bibliography and References:
BOITEAU P. 1942. — Plantes nouvelles de Madagascar. Bulletin de l’Académie malgache XXIV: 88-90.; Bosser J. & Lecoufle M. 2011. — Les orchidées de Madagascar. Biotope, Collection Parthénope, 496 p.; Cribb E & Hermans J. 2009. — Field Guide to the Orchids of Madagascar. Royal Botanical Gardens, Kew, 456 p.; Du Euy D., Cribb E, Bosser J., Hermans C. & Hermans J. 1999. — The Orchids of Madagascar. Royal Botanic Gardens, Kew, 376 p.; Hermans J., Hermans C., du Puy D., Cribb P J. & BOSSER J. 2007. — Orchids of Madagascar. Second Edition, Royal Botanic Gardens, Kew, 398 p.; Hervouet J.-M & Guérin J.-C. 2011. — Madagascar, the orchids of Ambondrombe, the mountain of the Dead. Proceedings of the 20th World Orchid Conference, 13-20 November 2011. Singapore Botanic Gardens, Singapore: 399-404.; Perrier de la Bâthie H. 1939. — Orchidaceae, volume 1, in Humbert H. (ed.), Flore de Madagascar et des Comores, 49. Antananarivo, Madagascar: 16-40.; Perrier de la BÂthie H. 1951. — Orchidées de Madagascar et des Comores. Nouvelles observations. Notulae Systematicae 14 (2): 138-165.; RICHARD A. 1828. — Monographie des Orchidées des Îles de France et Bourbon. Imprimerie de J. Tastu, Paris: 43-44.; SZLACHETKO D. L. 2000. — Gynostema Orchidalium I. Acta Botanica Fennica 169: 129, 130.