Mammillaria erythra forma ruber REP 879.
Mammillaria erythra f. rubber (Now referred to M. mystax) is an attractive plant with very dark carmine flowers and a characteristic epidermis, which clearly differentiates it from Mammillaria mystax.
 

Description: Mammillaria erythra f. rubber (Now referred to M. mystax) is an attractive plant with very dark carmine flowers and characteristic epidermis, which clearly differentiates it from Mammillaria mystax.  Furthermore M. erythra has a solitary growth and a different body size.
Stem: Dull green, that reddens in full sun, globose with an almost a flat-topped appearance, 40-90 mm high and 70-300 mm in diameter.  The apex is sunken and wooly.  With latex.
Roots: Fibrous.
Tubercle: Pyramidal, firm, 4- angled, keeled, 15 - 18 mm long and 12 - 15 mm wide.  The axil has white wool and 3 - 10 short, glassy white with reddish brown tip, straight bristles
Radial spine: 0 - 8, needle-like, straight, smooth, whitish to brown, 2 - 6 mm long.
Central spine: 1 - 4, the uppermost the longest, needle-like, straight or slightly curving, with thicker base, glassy brown with black tip, up to 50 mm long.
Flower: Dark carmine, funnel-form, 18 - 22 mm long and 10 - 12 mm in diameter.
Fruit: Dark carmine, club-shaped, 10 - 15 mm long and 3 - 5 mm wide, it ripens 5 months after flowering.
Seed: Brown, flattened keeled, 1,2 mm long and 0,8 mm wide. Hilum subbasal.
Blooming season (Europe): Mainly in spring (and sometimes other times of year).  It doesn't bloom frequently.
 

Scientific name:  Mammillaria erythra  f. ruber
In: Die gattung Mammillaria nach dem heutigen stand meines wissens 1987; Herausgeber AfM, 1987: 180 - 182 , 199 und 222.

Origin:  The “rubber” form has been collected by Werner Reppenhagen (Field number REP 879) at Tecamayuca, Veracruz, Mexico at an altitude of 1400m

Habitat: On crevices filled with humus, in lava rocks.

Conservation status: Listed in CITES appendix 2.

Synonyms:  

• Mammillaria erythra

• Mammillaria ruber

• Mammillaria mystax

• Mammillaria neomystax

• Mammillaria mutabilis

• Mammillaria huajuapensis

• Mammillaria crispiseta

• Mammillaria  atroflorens

• Neomammillaria mystax

• Cactus mystax

• Mammillaria  mystax var. huajuapensis

Cultivation: Requires excellent drainage provided by porous cactus soil.  We suggest repotting every 2-3 years. Prefers bright light and ample airflow. Outside full sun or afternoon shade, inside it needs bright light, and some direct sun. Water the soil abundantly when it is dry to the touch.  Allow soil to drain thoroughly before watering again.  Do not water much in the winter (Rot prone). Protect from frost. But it's hardy to -5°C if kept dry.  During the beautiful season enrich the soil using a fertilizer rich in potassium and phosphorous, but poor in nitrogen, because this chemical element doesn’t help the development of succulent plants, making them too soft and full of water.

Propagation: Direct sow after last frost as it usually don't braches.