Closely related to Mammillaria, just like Coryphantha is (these three make a loving family) that lives  in the same area (most families do) of southern USA and of Mexico (except for some Mammillarias that went to live in Central and even South America). They have “nipples” or mammillas instead of ribs (like Mammillarias and Coryphanthas) and they flower from the apex (unlike Mammillarias and like Coryphanthas) and they are small (like some Mammillarias) with small flowers. One should pay attention to ensure they are potted in a very open mix and that they are neither overwatered nor overpotted.