Biznaga prolifera. AKA Texas Nipple Cactus. Although the politically incorrect name has been ditched, it, and the genus name "Mammillaria" describe the plant well.
I got a few pieces from a lady who had it 50 years. I put them in a Styrofoam cup that I then put under the sink to catch a slow drip in the drainage pipes. I forgot about it entirely. A year later, I found it, completely in the dark and the drip had long since dried up. It looked like it might be alive, so I planted it, and it grew like crazy, and makes lots of seeds every year. I gave it to a friend, and now I just visit it to collect seeds.
