The Genus Maihuenia was first described by Karl Schumann in 1898. The plants are native to the South Western part of South America in the Mountains of the Andes, in Chile and Argentina from sea level to 2400 m. altitude. The Genus is a evolutionary distinct group of plants and is placed in it's own subfamily Maihuenioideae.

There are only two valid species in the Genus:

Maihuenia patagonica (Philippi) Britton & Rose 1919
Maihuenia poeppigii (Ottp ex Pfeiffer) Philippi ex. K. Schumann in 1898

Synonym names:
Maihuenia albolanata F. Ritter 1980 = Maihuenia patagonica
Maihuenia andicola H. F. Comber 1928 = Maihuenia patagonica
Maihuenia brachydelphys Karl Schumann in 1898= Maihuenia patagonica
Maihuenia cumulata F. Ritter 1980 = Maihuenia patagonica
Maihuenia latispinus F. Ritter 1980 = Maihuenia patagonica
Maihuenia philippi (F. A. C. Weber) K. Schumann 1898 = Maihuenia poeppigii
Maihuenia tehuelches Spegazzini 1902 = Maihuenia patagonica
Maihuenia valentinii Spegazzini 1902 = Maihuenia patagonica
Opuntia caespitosa Poeppig 1835 (illegitimate name, not Opuntia caespitosa Rafinesque 1830) = Maihuenia poeppigii
Opuntia maihuen Gay 1847  = Maihuenia poeppigii
Opuntia patagonica Philippi 1863 = Maihuenia patagonica
Opuntia poeppigii Ottp ex Pfeiffer 1837  = Maihuenia poeppigii
Pereskia philippi F. A. C. Weber 1898 = Maihuenia poeppigii

Cultivation:
I have only had experience with this Genus for 10 years or so, and I have had the best success in my unheated greenhouse. Of the two species Maihuenia poeppegii is the hardiest species and it can survive outside in the garden on a sheltered place, but only if it's protected with a sheet of glass during the wintertime. Maihuenia patagonica does not survive for long in the garden, nor in an frame, but is very good for a large pot which can be stored inside an unheated greenhouse during the winter. The best is if grow in a bed inside an unheated greenhouse all year around.

Closest relatives:
None, but in earlier literature they were supposed to be related to Opuntia and Pereskia and Pereskiopsis, but DNA sequencing by Robert Wallace's lab in Iowa, U.S.A. of this group has determined that Maihuenia is not related to any of those.

Other notes about the Genus
None at the moment.

Photos:

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Maihuenia patagonica (BMJ #571B)
Grown from seeds bought at Christa's Cactus, seeds collected at Napea Zopalo, Argentina, seeds sown 1993.
First flower that I ever have seen on this species, June 3rd. 2004.
Grown in a raised bed inside an unheated greenhouse, with lots of ventilation..

Maihuenia poeppigii
Does best if it's protected with a sheet of glass during the winter!

I'm looking for a picture of the plants in habitat, if you have any that I may use please let me know.

Literature:
Anderson, Edward F., The Cactus Family, (2001) pg. 398-399
Leuenberger B. E., Maihuenia  - Monograph of a Patagonian genus of Cactaceae. Botanische Jahrbücher für Systematik, Pflanzengeshichte und Pflanzengeographie 119:1-92 (1997).

Links to other sites about Maihuenia:
Cacti from Neuquén, Argentina by Joachim Saul (availiable with both English and German text)