Mammillaria is one of the larger genera in the Cactus family, and one of the most variable.
The three most common types of mutation are crests, monstrose growth and variegation. All of these are mutations of the cellular structure.
A genus of cacti from the dry coastal deserts, particularly the Atacama Desert, of northern Chile.
Astrophytum (Bishop's Hat) is a (mostly) star shaped cactus from north/central Mexico and southern Texas, USA.
Ariocarpus is one of the treasures of the world of Cacti, from Central to Northern Mexico and Southern Texas.
From Mexico and Southern US. Usually having heavy spination and relatively small flowers, the fruits are copiously woolly. Includes the "Golden Barrel Cactus".
Melocactus are recognizable by the large cephalium that develops on mature plants. They are native from Brazil to Southern Mexico, including the Caribbean.