Rebutia Clumping plants with large pink and orange flowers from the lower stem.
 

NOTE: The following description of this species is based on Rebutia ritter 1977 sensu stricto (Collector's number referred here is FR 1118). But plants in circulation under this name do not match up exactly with Ritter's description, most noticeably in their clearly defined ribs, unlike the much less defined ribs required by Ritter, and plants matching up to Ritter's description have not appeared in cultivation.
But if we consider the numerous synonyms, form and variety of this very variable taxon (Rebutia pygmaea), all the value of size, colour, flower and spines characteristic can change considerably.

Description: Very small clumping geophytical cactus.
Stem:  Individual heads globose, later columnar, with green body colour
Ribs: Loosely defined.
Areoles: Small, to 0.7mm long, with white wool.
Radial spines: Very thin, numbering 7 to 10, pectinate, about 2mm long, white, red-brown at the base;
Central spines: Absent or occasionally one similar to radials.
Flowers: Intense vermilion to scarlet, stigma yellowish-white.
Roots: tap root:

Photo of conspecific taxa, varieties, forms and cultivars of Rebutia pygmaea . (This taxon has lots of synonyms (like most Lobivia) whit several controversial varieties and subspecies):

• R. pygmaea
• R. pygmaea v. diersiana (pygmaea)
• R. colorea (pygmaea)
• R. gracilispina (pygmaea)
• R. haagei (pygmaea)

Rebutia   gracilispina     
  
Original description: F. Ritter, Kakteen und andere Sukkulenten, 28: 4; 76, 1977  / 28 (4): 76, 1977   
Type specimen: FR 1118 (deposited at Utrecht).

Scientific name:  Rebutia pygmaea (Fries) Britton & Rose.

Origin:  Reported from Bolivia, Potosi, Sud Chicas, Mai Paso, at 4,000m (10,200ft) altitude, where it is scarce.

Conservation status: Listed in CITES appendix 2.

• Rebutia gracilispina Ritter 1977 
• Mediolobivia pygmaea gracilispina
• Digitorebutia gracilispina  (hort.)
• Mediolobivia gracilispina   (hort.)
• Digitorebutia pozri - colls. FR 1118 
• Rebutia pygmaea var. gracilispina  (hort.)
• Rebutia pygmaea 'gracilispina'   (Pilbeam)
• Rebutia pygmaea subsp. gracilispina  (hort.?, sensu P-M & Anderson)
• Rebutia forma pygmaea haagei orurensis ?
• Rebutia haagei gracilis ?
• Rebutia pectinata gracilis ?
• Rebutia torquata

Cultivation:  It is easy to grow and recommended for beginners. Prefer gritty, porous mix with a pH slightly on the acidic side. Full sun to light shade, Water regularly in summer but do not overwater (the root system is rot prone) and allow the pot to dry out between waterings. Keep dry in winter Hardy to -4°C it will take low temperatures when dry and do require a winter rest period. This species will occupy a small flower pot comfortably and remain a manageable sized house plant.

Pest and disease: This plants are subject to mealy-bug attack and to fungus and rot problems brought about by overwatering and high humidity.

Note: It would appear that in cultivation they grow larger and cluster more vigorously than in habitat.

Propagation:  seed or offsets.