Mamillaria furfuracea Watson, Proc. Amer. Acad. xxv. 150
(1890).
Very closely related to prismaticum; but triangular portion of
tubercle acuminate and shorter, having an irregularly mamillate
upper surface, and the acumination ending abruptly in a
cartilaginous depression containing a tomentose pulvillus:
flowers 2.5 to 3 cm. long, white or pinkish, the sepals brownish.
Type, Pringle 2580 in Gray Herb.
At Carneros Pass, Coahuila.
Specimens examined: Coahuila (Pringle 2580 of 1889).
The type of this species was not among the collections received
from Cambridge, but a specimen of the same distribution from the
National Herbarium shows tubercle dimensions different from those
recorded in Dr. Watson's description. In that description the
triangular terminal surface is said to be "about an inch broad by
one-half inch," which is decidedly different from the equilateral
surface of the tubercle of prismaticum. In the National
Herbarium specimen of furfuraceum, however, of the same
distribution, the surface is almost equilateral, measuring 15 mm.
long by 18 mm. wide at base. Without the acuminate upper portion
the breadth of the triangular portion would be about double its
length. The lower rim of the cup-like depression which
terminates the tubercle and contains the pulvillus is sometimes
slightly prolonged into a tooth, which in prismaticum becomes the
sharp tip of the tubercle. The "minutely
furfuraceous-punctulate" character of the tubercle is common to
all the species of Anhalonium I have seen, and simply represents
the external openings of the remarkably long cuticular
passageways to the stomata.