Looks like Gymnocalycium anisitsii.

It occurs to me that this cactus looks like a large version of my Gymnocalycium damsii var. rotundulum. I assumed they stayed small though. I've got two of them, one of which is pictured below. Bugger, I've got a feeling I've got another one of those darned variable Gymnos!

Bill, feel free to add your 2¢. I bet you've got one of these in your collection. For reference, the cactus in question is about 3" in diameter.

Amy

{{gwi:481408}}

G. anisitsii is a synonym of G. damsii var rotundifolium :)

I kinda thought they stayed small also, but that's surely what is looks like. :-)
Bill in SC