Rebutia senilis was discovered by C. Backeberg in Salta, Jujuy, Argentina and first described in Der Kakteenfreund 1, p. 123, by C. Backeberg in 1932. I have been unable to locate a copy. However the description was re-published by Backeberg three years later, in Backeberg, C. & Knuth, F. M. (1935), Kaktus ABC, pp. 277 & 416. Gyldendalske Boghandel-Nordisk Forlag, Copenhagen, Denmark. As well as the nominate, four additional varieties were described.

Here is the text of the description in the original Latin with my own English translation below. You should look up the original for more details in Danish and a black-and-white photograph.

Rebutia senilis Bckbg. 1932

Depresso- globosa, ad 25 cm alta sed saepius 6 cm alto 7 cm lato, laete viridis, in tubercula parva spiraliter ordinata divisa, areolis 1 cm inter se remotis; aculeis setaceis 35—40, conformibus, tenuibus, vitreis, ad 3 cm longis; flore ad 5 cm longo 4 cm lato, laete carmineo, ovario pallide rubro. — Argentina: Salta, alt. 2500m.

var. aurescens Bckbg. var. nov.

Aculeis setaceis longis, densis, centralibus luteolis.

var. breviseta Bckbg. var. nov.

Aculeis setaceis 4—7 mm longis, lateraliter intertextis, rigidioribus, nonnullis luteolis.

var. Stuemeriana Bckbg. var. nov.

Floribus fulgenter lutescente lateritiis, fauce saepe luteolis.

var. lilacino-rosea Bckbg. var. nov.

Floribus lilacino-roseis colores iis R. violaciflorae similibus.

English translation

Rebutia senilis Bckbg. 1932

Flattened spherical, usually up to 7 cm wide and 6 cm high, but in some cases up to 25 cm long and then club-shaped. Light green; spiral tubercles; areoles 1 mm apart; c. 35—40 spines, fine, translucent, up to 3 cm long, central spines and marginal spines alike. Flower up to 5 cm long and 4 cm wide, light carmine red.

– Argentina: Salta, 2500 m H.

Differs from R. xanthocarpa by having larger flowers.

Var. 1st aurescens Bckbg .: long, dense spines; between the marginals, yellow central spines protrude, giving the plant a yellowish tinge.

Var. 2. breviseta Bckbg .: Lateral spines intertwined, but only 4-7 mm long, rather stiff, strong, more compressed spines, including some yellowish.

Var. Stuemeriana Bckbg .: Flowers glitteringly luminous brick red, often with a yellowish throat.

Var. 4. lilacinorosea Bckbg .: The flower lillac pink, similar to R. violaciflora.