In their monumental work "The Cactaceae," BRITTON and ROSE (I9) record 1235 species belonging to the three tribes which constitute the family of the Cacti. The actual number of the species must be con siderably higher. Cacti occur frequently in tlie more arid and less accessible regions of the American Continent, nearly always within very narrow and definite borderlines. The habitat of a species is in many instances a single valley located in a remote, uninhabited region of the Cordillera. Thus the collection of flowering specimens fit for botanical identification is some times extremely...