Family Cactaceae > Subfamily Cactoideae > Tribe Cacteae > Subtribe Cactinae.

There are 170 to around 200 species almost all from Mexico.
It is one of the largest Cactaceae genera with many species, varieties and cultivars.
Classification is still not settled with species moved in and out from similar genera.

The ball-shaped to cylindrical plants can be solitary but commonly form large clumps.
They can be up to 40 cm high and 20 cm wide and are covered in variously shaped tubercles.
The areoles are unique to this genus being split into 2 parts.
The spines are on the top the tubercles and the hairs and bristles are at the base.
Sometimes the base has no hairs or bristles.
The spines vary greatly in number, form, stiffness and colour.

The funnel-shaped flowers arise from the basal part of the areole.
They often form a ring around the crown.
Flowers vary in size from under 1 cm to over 4 cm long.
They can be red, pink, yellow, greenish or white.
There is often a darker stripe down the tepal centres.
The mostly club-shaped fruit are often red but can be white, yellow, green or purplish.