This is in flower today, but not quite warm enough to get the flowers fully open. This is a plant I raised from Koehres seed in 2006, but this is it's best display so far!
Hunt says that this is only a neotonic (juvenile form which remains so) form of Mammillaria lasiacantha. I think that the flower position and its later flowering (in general) does certainly distinguish it, as well as the spination, so I have kept the name rather than lumping it in with the rest. Even on the bench it looks different, but clearly related.
But what do I know as an amateur?!