6, rigid, 4 to 6 mm. long, the upper one stouter than the rest
and sometimes shorter, reddish-brown and horny, straight or
slightly curved, the remainder straight and white with dusky
tips; central spine stout and horny, reddish-brown, 7 to 10 mm.
long: flowers greenish-white or tinged with red: fruit unknown
Type unknown.

Entirely Mexican, reported from Chihuahua to Saint Luis Potosi.

Specimens examined: San Luis Potosi (Gregg of 1848; Parry 268;
Eschanzier of 1891): Chihuahua (Wislizenus of 1846-47; also
Chihuahua specimens cultivated in the Jacoby Garden in 1856 and
1857).

The variations observed in this species do not seem sufficient
for the establishment of varieties. The type form seems to have
been globose, with 4 radial spines and a stout central one. The
depressed forms with 6 radials and a more slender central
represent var. spinosior Lem. (M. depressa Scheidw.); and the
subcolumnar forms with 6 radials (the upper one of which is
somewhat curved) and a stout strongly hooked central represent
var biuncinata Lem. (M. bihamata Pfeiff.) Such combinations of
characters, however, do not hold, as any one of the plant body
forms may display any one of the spine characters referred to.

B. Tubercles terete.

* Central spines none: mostly simple globose plants, with very
numerous straight whitish setaceous radials.