Mammillaria heyderi, a frost-tolerant species, is quite widespread across higher elevations of the Southwest deserts, from Arizona to Texas, usually between 3,000 and 6,000 feet elevation but extending all along the Rio Grande to the Gulf of Mexico. The cactus has an unusual shape, remaining nearly flat against the ground, instead growing outwards and downwards, but never becoming much more than six inches across.
The green stem bears prominent tubercles, each topped by 8 to 18 short, whitish (brown at the tip) spines which do not obscure the body of the plant; all are radials apart from one slightly longer central spine. This is the only Southwest cactus with milky sap. Five varieties are recognized (bullingtoniana, hemisphaerica, heyderi, macdougalii, meiacantha), distinguished by flower color and spine characteristics.
Heyder pincushion cactus, little nipple cactus, cream pincushion
Southeast Arizona, much of New Mexico, south Texas (along the Rio Grande); also Colorado and Oklahoma
Flat, disc-shaped, solitary
Scrubland, grassland, pinyon-juniper foothills
Pink, yellow or cream, with a darker stripe underneath