Mammillaria 
Also called bird's-nest cactus, golden-star cactus, old-lady cactus, pincushion cactus, snowball cactus
(Mammillaria species)

Mammillaria cacti can be round- or cylinder-shaped, and soft- or sharp-spined.  Choose several of the many species to form a fascinating plant collection that requires minimal attention.  These cacti sometimes flower in early spring in colors that vary by species.

Light
Place in high light (southern exposure) year-round.  Augment low light in winter with artificial light for 16 hours per day.

Water
Allow soil surface to dry out between thorough waterings.  Water even less in winter, offering just enough to keep plant from withering.

Temperature
Maintain average temperatures except during dormancy in winter.  Then let temperatures drop slightly.

Comments
Use a sandy medium.  Feeding is unnecessary.  Propagate by seed or offsets.