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THE CYCADs are the most primitive of the living gymnosperms and represent the surviving remnants of a line reaching back through the Mesozoic into the Paleozoic era.Nine genera, containing less than one hundred species, are recognized.Four of the genera occur exclusively in the western hemisphere and the other five in the eastern.Of the western genera, Zamia, with twenty-six species, ranges from southern Florida to Chile.Microcycas is a monotypic genus and is found only in western Cuba.Dzoon, with three species is found only in southern Mexico, and Ceratozamia with two species has about the same range as Dioon.
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Sax et al. (1934) studied this question.