++ Stems depressed-globose to ovate.
Radials 4 to 6, rigid.
uncinatus (9).
Radials 8 to 12.
wrightii* (15).
Radials 15 to 30.
grahami* (19), eschanzieri (21).
Radials 50 to 60.
barbatus (18).
**** Central spines more than one, and none of them hooked.
+ Slender or sometimes stout cylindrical plants, branching at base: Lower Californian.
brandegei (3), setispinus (24), halei (25).
++ Depressed-globose to ovate and stout cylindrical.
++ Radials few (3 to 12) and rigid: Mexican.