Found in the Pilcaya Mountains of Guerrero, Mexico, Mammillaria spinosissima ssp. pilcayensis is a solitary cylindrical cactus to 20 inches long and 2 inches thick. I didn't find any more info about it but I would guess the plant becomes prostrate over time. It might be interesting to plant several in a hanging pot. It's reported the flowers are a deep pink.
     I haven't come across this one before, I like the white spines. The spines though sharp, are very flexible, so the plant is easily handled. I went back and got two more and planted all three together to see if they'll trail out of the pot. Let's wait and see.