My favourite of the Volunteer Park Conservatory glasshouses was the cacti and succulents house, since I was pleased to see what could be accomplished with growing these typically sun-loving plants in an area of the world not known for its sunshine.
Mammillaria prolifera is native to Texas, Mexico, Cuba and Hispaniola. A variety in Texas and northern Mexico, Mammillaria prolifera var. texana, is recognized by the Flora of North America. The FNA also gives a hint to identifying the species (as long as one is north of the USA-Mexico border: “the hairlike radial spines of Mammillaria prolifera provide an instant means of identifying this species, even without reproductive material”. Common names include Texas nipple cactus (note the red fruit in the background or see this photograph), little candles, and silver cluster cactus.
If search engine results are anything to go by, this species is relatively common in cultivation. Growing information is available via the RHS: Mammillaria prolifera.