This is one of the largest growing and the heavier spined specie in the gymnocalycium group.

Description: Description:  They have been described as solitary, but they can also form groups with age.
Stem: Usually globular up to 30 cm tall and 18cm across. Grass green to olive green
Ribs: About 9-12 which consist of flattened protuberances, each of which finishes with a large elliptical, densely furred areola length 12 mm.
Radial spines: 9-15 (-20) up to 4 cm long; Varying from brownish to greybut those near the apex are mostly reddish-brown.
Central spines: 4-6 (-11) resembling the radial ones.
Flowers: White with a reddish throat. The diameter and length are identical and are equal to 5 cm. Seedlings bloom at the age of 5-7 years.


Note: Seems to be a pretty variable plant you might get some other names which are effectively the same plant.... and also not sure all plants identified as this are really this.

Conservation status: Listed in CITES appendix 2.

Gymnocalycium valnicekianum Jajo 1934 (valniceckianum)

Scientific name:  Gymnocalycium mostii (Gürke) Britton & Rose 1918
 

Origin & habitat: Argentina (Córdoba) This plant grows among hight grasses in the mossy cracks and cavities in rocky cliffs.
The plants in habitat are each one different from its neighbours! Some had strong spines others weak, curly, straight, long or short ones and in all different combinations.
 

Large white flowers with a reddish throat from mid spring to early summer.

Cultivation: summer grower and pretty easy, water regularly in summer, keep rather dry in winter, but can handle excessive water to few water, best for half-shade but grow well in full sun and full shade too (but in full sun turns an uneven brownish red colour). style="font-family: Arial"> Frost Tolerance: Hardy to -12C.

Seed Collecting: Permit fruit to ripen, fruit must be significantly overripe before harvesting seed; clean and dry seeds