(2) Those of the Department of Agriculture, including the results
of several recent explorations, for the use of which I am
indebted to Mr. Frederick V. Coville.

(3) Those of the Gray Herbarium at Harvard University, which Dr.
B. L. Robinson kindly placed at my disposal.

(4) Those of the California Academy of Sciences, notably rich in
forms from Lower California and the adjacent islands, kindly
loaned by Mr. T. S. Brandegee.

(5) Those of Dr. Louis Eschanzier, of San Luis Potosi, Mexico,
who send a large series of Mexican forms collected in 1891.

(6) Numerous small sets from different correspondents, who have
given both time and material in aiding the work.

It is needless to say that Dr. George Engelmann, the great
pioneer student of this difficult family, has opened the paths in
which we must follow, and it was exceedingly unfortunate that he
was not able to complete the final revision that he had in mind.

The difficulties which beset the critical study of this group can
not be easily exaggerated. Such scanty material as has been
collected has been for the most part very incomplete, consisting
of plant bodies without flower or fruit, flower or fruit without
plant bodies, and bunches of spines without either. The species
are displayed also in the most inaccessible regions, and their
culmination is found in the still poorly known regions of Mexico.