Rebutia minuscula was first described in Monatsschrift für Kakteenkunde (1895):5, pp. 102 – 105, by E. Werdermann. It is the type species for the genus.
Below is my own English translation of the main points of the description, which was published in German. You should look up the original for extensive further notes in German and a fine line drawing.
Rebutia minuscula K. Sch.
A new genus of cacti.
By K. Schumann.
Body somewhat more than hemispherical, indented and deepened at apex; 2.5 cm high and almost 4 cm in diameter. Tubercles fresh green with greyish cast, very flat; at the base almost circular, not rising 1mm above the surface of body and 5-6 mm in diameter when full grown; almost all in longitudinal rows, seldom crooked; 21 ribs on described specimen. Areoles a little over 1.5 mm in diameter, circular or almost elliptical, on apex or slightly back on tubercle with a shallow depression behind the apex. Younger areoles up to edge of apex depression are completely unreinforced, somewhat reminiscent of Echinocactus concinnus Hk. fil., the others with a little, short wool around 30 spines, the longest of these 7-8 mm long; radiating, bristle-shaped, softish; mostly white, some upturned slightly yellowish.
Flowers funnel-shaped, somewhat curved, arising numerously at base of the body; 13 on described specimen; 3.5 cm long, of which 2.5 cm on ovary and tube. Ovary 2 mm in diameter, almost spherical enclosing numerous ovules attached to long seed-carriers: prominent, pink-red with several dark red, 1 – 1.5mm long scales. The similarly dark red flower tube is covered with 2 or 3, 6 mm long leaves, petals about 1 cm long, lanceolate, finely pointed, the outside gradually darkening from the base, inside fiery, shiny red, merging into yellow throat. Stamens attached to tube 9-15 mm above base, 12-15 mm long, yellow, anthers barely 1 mm; variable number, 7 to 34. Pistil 24 mm long, at least later yellow at the top, four stigmas 2.5 to 3 mm long. The ripening fruit is rose-red.
According to a kind message from Mr Rebut in Chazay d’Azergues, this plant probably comes from Argentina.