As has been already remarked, the occurrence of genuine Cycads in the Carboniferous vegetation has not been demonstrated, and the same holds good of all the Palaeozoic floras. True Cycads, therefore, so far as known, make their first appearance in the Trias, at the commencement of the Mesozoic period, where they are represented by the genera Pterophyllum, Zamites, and Podozamites.