The form "longispinum" distinguishes  from the type form for its slightly longer and stiff spines.

Description: Solitary cactus. The form “longispinum” has longer and stiffer spines but is practically identical to the normal form in stem and flowers characteristics.
Stem: Deep glaucous-green to grey-black, discoidal that became elonged with time 10-25 cm tall, up to 12 cm in diameter, the apex is slightly woolly.
Ribs: 10-16 straight, divided in well distinct angular ribs.
Areoles: Whitish-grey, elonged.
Radial spines: 5-7 25-35 (45) mm long slightly bent against the plants body, clear brown with a darker base when young, that became whitish-grey with a black base as they age.
Central spines: 0-2 very similar to the radials.
Flowers: Infundibuliform in the nearest periphery just around the apex 45-60 mm long, 60-65 in diameter. Tepals ivory white with a brownish mid rib or occasionally reddish, throat greenish with a basal reddish area. Style white-greenish, stigma white. Anthers cream coloured.
Fruit: Globate or fusiform, 20-40 mm long green with white bordered scales.


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.

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
 

Conservation status: Listed in CITES appendix 2.

Scientific name: Gymnocalycium  gibbosum (Haworth 1812) Pfeiffer ex Mittler 1844 "longispinum"

Common Name: Black Chin Cactus

Origin & habitat: This species is widely distributed in the mountains of Argentina from the south of the Buenos Aires province to Patagonia and show a great variability in morphological traits.

Synonyms:

• Cereus reductus,
• Echinocactus gibbosus,
• Gymnocalycium reductum,
• Gymnocalycium gerardii,
• Cereus gibbosus,
• Cactus reductus,
• Cactus gibbosus,
• Gymnocalycium chubutense
   

G. gibbosum is the type species of the genus Gymnocalycium
 

The flowers are pearly white from mid spring to early summer.

Photo of conspecific taxa, varieties, forms and cultivars of Gymnocalycium gibbosum.

• Gymnocalycium gibbosum
• Gymnocalycium gibbosum f. longispinum
• Gymnocalycium gibbosum var. brachypetalum
• Gymnocalycium gibbosum var. nobile (= reductum)