Mammillaria Senilis

Mammillaria senilis is a species of cactus native to high-altitude areas of Mexico. This species is typically found growing on moss covered boulders in pine forests. As a mountain species, this cactus prefers slightly cooler temperatures, and can be quite frost-resistant if kept dry. High light levels are needed for mammillaria senilis to develop proper spines and flower.
 
This species is considered very ornamental, and physically described as a cotton ball with very sharp hooked spines, with many white radials as well. Mammillaria senilis branches basally to form clumps as it grows. This cactus flowers in Feburary and March, and features large, showy, red blooms. While the flowers are usually red-orange, they can also be yellow or rarely white to pink.
 
Note: this species is one of the more difficult species of Mammillaria to keep and grow. It has a small root system, making it extra sensitive to over-watering. It also needs lots of light and plenty of air flow. This slow-growing species needs to be watered regularly in the summer, but letting the soil dry out completely between waterings. During the winter, mammillaria senilis needs to be kept very dry to prevent root rot. It is reccomended to use very well draining soil for this species, including extra perlite or pumice to achieve that extra draining levels in the soil.
 
[sourced from: Llifle - Mammillaria Senilis]