From Mexico Mammilaria matudaea is a clumping globular cacti with stems to 2 inches around and 12 inches tall. One variety (serpentiformis) eventually falls over and keeps on growing parallel to the ground. It's common name is "Giant Snake". They are apparently variable as far as flower color. I've seen yellow flowers and this one of mine has purple ones.
     I'm not sure if mine is a serpentiformis or not. I guess only time will tell.