Thank you both. I agree it is probably Mammillaria. There's no way to see through the spines to the body of the plant. No hooks. It is quite small (the rocks are pumice chunks in the range of a quarter to a half an inch). This plant was a gift from a traveler, bought at a nursery. I think probably not elongata just because of the size (and the ease of propagating that plant from cuttings). It's hard to know if it's just young or really small (maybe both) but given it's flowering, I would imagine the second. I will have to spend some time looking at the different Mammillarias to try to sort them all out.
