Origin and Habitat: Endemic to a small region in Central Mexico at the border of the states of Puebla and Oaxaca (extent of occurrence is approximately 2,500km2).
Altitude: 800 to 1150 m.
Habitat: Mammillaria crucigeraSN|1835]]SN|1835]] grows on vertical cliff edges clinging in clefts and crevasses in the rock often in mostly pure gypsum. The habitat where the species grows is highly specialised and fragile. It grows on cliffs that are easily eroded and as a result individuals have high mortality, therefore there is a continuing decline. The area is mainly covered with dry tropical forest and in the enclaves xerophytic vegetation is dominant. Goats searching for food on cliffs can cause disturbances of varying intensities. The species is also affected by herbivory of a beetle that eats the tissue from the inside killing the individuals.

• Mammillaria crucigera Mart.
• Cactus cruciger (Mart.) Kuntze
• Cactus curvispinus (Otto) Kuntze
• Mammillaria curvispina Otto
• Mammillaria discolor var. curvispina (Otto) Salm-Dyck

• Mammillaria crucigera Mart.
• Cactus cruciger (Mart.) Kuntze
• Cactus curvispinus (Otto) Kuntze
• Mammillaria curvispina Otto
• Mammillaria discolor var. curvispina (Otto) Salm-Dyck
• Mammillaria crucigera var. grandinosa Lizen et al.

• Mammillaria crucigera subs. tlalocii (Repp.) D.R.Hunt
• Mammillaria crucigera var. tlalocii (Repp.) Lizen et al.
• Mammillaria tlalocii Repp.
• Mammillaria buchenaui Backeb.
• Mammillaria crucigera subs. tlalocii f. caespitosa hort.
• Mammillaria tlalocii f. caespitosa hort.
• Mammillaria crucigera subs. tlalocii f. cristata hort.
• Mammillaria tlalocii f. cristata hort.

Description: Mammillaria crucigeraSN|1835]]SN|1835]] is a globular slow growing cactus with small spines, solitary or clumping almost always forking dichotomously at maturity. Two subspecies are recognized, the nominate and ssp. tlalocii (Repp.) D.R.Hunt.
Stems: Heads flattened globose to short cylindrical, curving pipe-shaped, up to 10 cm tall and 4-6 cm in diameter, ranging from olive green to grey-green to almost brown to almost purple.
Tubercles: Closely set, firm, not sharply angled, keeled with their axils full of white wool, Some latex is present in the growing season.
Areoles: Reddish-brown.
Radial spines: 22-30, finely needle-like or bristly, glassy white, to 2 mm long.
Central spines: 4 (-5), yellowish or brown, rigid, to 2 mm long.
Flowers: Small, funnelform, up to 1,2 cm, deep pink, rarely rising above the spines.
Blooming season: Flowers in late winter, early spring (In habitat it flowers between December and March)
Fruits: Pink to red.
Seeds: Small, brown.
Chromosome number: 2n=22, x=11.

Notes: This is one of the Mammillaria commonly called "Owl Eye Cactus", known for dichotomous branching (forking or dividing into two parts). Although dichotomous branching is not a common occurrence in cacti in general, it happens for some reason in this particular species. What is interesting about this cactus is that it began as a single head, one head became two, and so on. Other Owl Eye Cactus among others comprise Mammillaria karwnskiana, Mammillaria formosaSN|15832]]SN|15832]], Mammillaria microthele and Mammillaria perbella.

Bibliography: Major references and further lectures
1) David Hunt, Nigel Taylor “The New Cactus Lexicon” DH Books, 2006
2) John Pilbeam (1999) “Mammillaria The Cactus File Handbook” Nuffield Press.
3) Edward F. Anderson “The Cactus Family” Timber Press, 2001
4) James Cullen, Sabina G. Knees, H. Suzanne Cubey “The European Garden Flora Flowering Plants: A Manual for the Identification of Plants Cultivated in Europe, Both Out-of-Doors and Under Glass”Cambridge University Press, 11/ago/2011
5) Nathaniel Lord Britton, Joseph Nelson Rose “Cactaceae: Descriptions and Illustrations of Plants of the Cactus Family” vol. 4 The Carnegie Institution of Washington, Washington 1923
6) Ulises Guzmán, Salvador Arias, Patricia Dávila: “Catálogo de cactáceas mexicanas.” Universidad Nacional Autónoma de México, Mexiko-State 20038)
7) Florencia Briones, Guadalupe Palomino & Armando Garcia “Chromosome analysis of Mammillaria supertexta, M. crucigera and M. haageana and their comparison with M. san-angelensis (Cactaceae)” in Caryologia: International Journal of Cytology, Cytosystematics and Cytogenetics Volume 57, Issue 3, 2004
8) Arias, S., Valverde, T. & Zavala-Hurtado, A. 2013. Mammillaria crucigera. In: IUCN 2013. "IUCN Red List of Threatened Species". Version 2013.2. <www.iucnredlist.org>. Downloaded on 23 March 2014.

Cultivation and Propagation: It is an extremely slow growing species (hence the rarity and high prices in the trade). Use pot with good drainage and a very porous mineral-based potting mix, keep dry in winter. Pot plants are quite wet-sensitively. Care must be taken with watering (Rot prone) and needs good drainage. Water sparingly during the growing season, keep very dry in winter. Feed with a high potassium fertilizer in summer.
Usually it is recommended to overwinter this plant in a bright and warm greenhouse with at least 8-10° C , but it proved to be quite frost resistant (if kept dry it hardy as low as -5° C)Outside full sun or afternoon shade, inside needs bright light, and some direct sun.
Some plants will offset readily, and clumps can be produced in a very few years.
Propagation: Direct sow after last frost, cuttings or grafting.