I'm sort of thinking Mammillaria melanocentra. I have one in my collection of baby Mammillarias that I gave that name. I'm off to find a decent picture to compare with yours.

Al

Here's my plant that I took the liberty of naming 'melanocentra'. I don't often do that without consulting our sitting committee of experts. It's in a 3 1/2" pot. I think there are similarities.

Al

{{gwi:486800}}

Hmmmm doesnt look like a mamm to me, more gymno or matucana like.

I think Al has nailed it... we can confirm it when it flowers, melanocentra has larger than normal flowers to 2cm x 2cm.

Here 'tis...
{{gwi:486801}}

BTW nice pic! CP

Classic M. melanocentra ssp melanocentra.

Thanks, WsG for the original post and more thanks to our loyal cactus ID guys. I'll sleep more soundly tonight knowing that one more Mamm has a valid name. I have about 8 more little ones waiting in the wings. I'll introduce them slowly for their christenings.

Al

And if I may I'll call it a M. Melanocentra but I too have one and am waiting for it to bloom or do something like cluster. If it proves to be a Melanocentra look alike then I'll have to rename it M. compressa. Until then it'll straddle the species fence. I believe the M. compressa should have red flowers and clusters heavily vs. M. Melano. which has pink flowers and is generally solitary. I'll have to wait till next Summer as will we all. :(