Mammillaria spinosissima. As you can see, this mammillaria is also blooming in a circle on the top of the plant. I think this is typical of all mammillaria varieties.
This is one of my unidentified mammillarias. It, too is making a halo of flowers around the top of the plant. At the base is anacampseros rufenscens, which blooms in the summer. It is similar to the first variety pictured, but notice the difference in the color of the spines and the shape of the petals on the flowers.