1. Origin and relationships of the cycads;Arnold;Phytomorphology,1953
2. Notes on the Upper Paleozoic plants of central New Mexico;Ash,1982
3. Morphology of vascular plants;Bierhorst,1971
4. The Living Cycads;Chamberlain,1919
5. The living cycads and phylogeny of seed plants;Chamberlain;Am. J. Bot.,1920