Even after duplicate names are sorted out, Mammillaria is the largest of several genera of diminutive cacti with about 175 species worldwide. The Sonoran Desert species are less than 6 inches (15 cm) tall, with closely-spaced areoles bearing many spines that obscure the body of the plant. The areoles are at the tips of long tubercles that are arranged in 2 spirals (one clockwise, the other counterclockwise). These tuberculate stems contrast with the ribbed stems of the genus Echinocereus. In addition, many mammillarias have hooked central spines, whereas no hedgehog does.