Mammillaria theresae minima
This is one of a few miniature species of Mammillaria which produce flowers which are very large in proportion to the plant. This is form minima an even smaller variant of the small species. Even with some lateral stems developing, This is only about 40mm across.
There are three unopened buds. I have further images, with those open, to post in the near future.
The stereos are crosseye.
Olympus EM-1, (aperture priority), Olympus 4/3 50mm f2 macro, 1/40 or 1/50 at f11 ISO 400, hand-held.