Gymnocalycium stenopleurum v. friedrichii is a small flowering barrel cactus from Paraguay.
While the common name for these Gymno’s is Chin Cactus and I prefer to refer to them as Gymnos, the name itself, Gymnocalycium, means Naked Calyx. You can see this in the flower buds, which if you could see the side view of this flower you would see a bit of the nakedness of the calyx. But I have hidden that NSFW side view from the internets. Or you could click here to see what I’m talking about.
This cactus is also one of the ones that are irridiated to create the bright colored “Moon Cactus” that get grafted onto small Echinopsis.