Here is the other California Mammillaria - dioica. To be precise M. grahamii is also known to grow in the state, at the small area where also Saguaros grow, at the only spot west of Colorado river.
Southern California is actually the northern tip of M. dioica distribution, the plant common in Baja California desert. This picture is taken at Cactus trail in Anza Borego State Park. Cactus trail is a wonderful place to visit for every cactus aficionado. Cactus plants there grow in abundance, several species of chollas, Opunitas, Mammillaria, Echinocereus and Echinocactus species. The best time to visit is mid to end of April, depending on a year.
Mammillaria dioica is another easy to grow plant, not very common in collections.