. Annual report of the Board of Regents of the Smithsonian Institution . genus Pereskia; (II) Opuntioidea?,composed of Opuntia and its allies; and (III)Cereoideae, divided into two tribes, (1) EcMnocacteae^ includingCereus, and its allies, Echinocactus^ Leuchtenbergia^ Melocactus,Phyllocactus Epiphyllum., Hariota^ and Rhipsalis; and (2) Mamil-lamece^ including Mamillaria^ Pelecyphora^ and Ainocarpus. With a few exceptions the genera and subgenera of Cactacese, astreated by early writers, are not sharply separated by definite many of them there are transition species which have charac
. Annual report of the Board of Regents of the Smithsonian Institution . genus Pereskia; (II) Opuntioidea?,composed of Opuntia and its allies; and (III)Cereoideae, divided into two tribes, (1) EcMnocacteae^ includingCereus, and its allies, Echinocactus^ Leuchtenbergia^ Melocactus,Phyllocactus Epiphyllum., Hariota^ and Rhipsalis; and (2) Mamil-lamece^ including Mamillaria^ Pelecyphora^ and Ainocarpus. With a few exceptions the genera and subgenera of Cactacese, astreated by early writers, are not sharply separated by definite many of them there are transition species which have character-istics of two genera or subgenera. Thus there are species of Echino-cactus very closely resembling certain Mamillarias (Coryphanthse) intheir structure, and the relationship of several of the columnar Cereiis not clear. Various authorities have divided the old genera ofOpuntia, Cereus, Echinocactus, and Mamillaria into groups, some-times regarding them as distinct genera, sometimes as subgenera, orseries. Many of these closely allied groups difi^er radically from. 5 Fig. 9.—Cactus seeds. 544 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1908. one another, as in the structure of the seeds, yet in some cases thereseem to be intermediate forms, and the flowers themselves are remark-ably similar. Notv/ithstanding this, it would be of gieat assistance tothe student of Cactacese to recognize many of the so-called subgeneraas genera^ not only for convenience of study, but also to show thesystematic connection between the members of each group. In assign-ing the various species to a particular genus it will often be difficultto decide to which of two it belongs, but as Schumann says, thisdifficulty can not be avoided unless all the Cactacese be combinedinto a single genus Cactus.^ In the following synopsis of Mexican genera, while following Schu-manns arrangement I shall include various genera of Opuntioidea3and Cereoidese established by Britton and Eose, including Pere-skiopsis and Cam
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