From Hidalgo Mexico, Mammillaria geminispina is a beautiful white spined clumper that can produce mounds 3 feet across in the wild. The Heads average about 4 inches in diameter. Spination varies from short to extremely long. The example I have has the longest spines I've seen on this cactus! Flowers are small and usually red or yellow.
     I picked this one out for its long spines and the way it offsets from the sides. I think it will make an interesting garden specimen. The picture below is of a clump of it growing on the grounds at Bachs Cactus Nursery where I bought it.