Coryphantha also has tubercles and its areoles are also dimorphic (spines at the tip of the tubercles, flowers at the axils)... differences being that Coryphantha flowers at the apex (mostly) instead of in a crown-like ring on earlier growth; and Coryphantha has grooved tubercles. The grooves are less evident on some plants (to my non-expert eyes) than others. Complicating things, some plants make grooveless and grooved tubercles, the latter appearing only later in life.
