Species: Gymnocalycium ragonesei & Gymnocalycium mihanovichii var. friedrichii
Scientific classification
Kingdom:Plantae
(unranked):Angiosperms
(unranked):Eudicots
(unranked):Core eudicots
Order:Caryophyllales
Family:Cactaceae
Subfamily:Cactoideae
Tribe:Trichocereeae
Genus:Gymnocalycium
From my collection
via Flickr https://flic.kr/p/2kaktvk