Mammillria microhelia

Mammillria microhelia is a popular plant, often available from garden centers. When grown in strong light conditions, it forms dense spine coverage, eventually clumping. Bonus pink-yellow flowers are easily produced in spring.
Mammillria microhelia is closely related to Mammillaria elongata, another very popular plant, and probably deserves a subspecies status, along with another relative Mammillaria mieheana. Mammillaria elongata was described almost 100 years earlier and that name would take the precedence.