Mammillaria perbella. There
are around 300 species of Mammillarias. Most are native to Mexico.
In their range you can find Mammillarias from sea level to high
elevations. They also vary in size from miniature 1 inch in diameter
(2.5 cm), to columnar types 1 foot tall (30 cm). And there are many
clumping types. The name Mammillaria comes from Latin for "nipple",
because the areoles (structures carrying the spines) are cone shaped. You have a choice of specimens.