G. ubelmannianum is a little and lovely species with beautiful flowers.
 

Description: Plants usually solitary at or forming clusters with age. Stems flattened globose, often pulling into the ground, glaucous grey-green, to 1 cm high and 7 cm in diameter. With s 8-12 ribs, forming distinct tubercles with chinlike protrusions and grooves between the areoles.
Spines: Spine clusters have a characteristic spiders-like shape, Spines (all radials) 5-7 chalky white, flexible, somewhat curved and often twisted, one pointing downward, 0.5-1.5 cm long.
Flowers: cream-white with a darker pinkish-reddish throat to 3.5 cm long and in diameter.
Fruits: broadly globose, green, to 6 mm in diameter.
 

The length of the spines is quite variable also in seedling of the same batch from 5 to 20 mm.

Scientific name:  Gymnocalycium ubelmannianum Rausch 1972
Published in:   Succulenta 51(4): 61-64, 1972

Origin:  Argentina (La Rioja)

Conservation status: Listed in CITES appendix 2.

Etymology: Named in honour of Werner Übelmann, a Swiss citizen that worked to discover new cactus species.

Flowers are cream-white with a darker pinkish-reddish throat

Cultivation: It is a summer grower species that offers no cultivation difficulties.  Water regularly in summer (but do not overwater ) keep rather dry in winter. Feed with a high potassium fertilizer  in summer. It is quite frost resistant if kept dry (hardy to  -5° C) Sun Exposure: Light shade.

Propagation: Direct sow after last frost. (seldom produces offsets).