Marratiopsida

Order of tropical ferns which date back to the carboniferous era. It is considered to be more primitive than the Polypodiopsida as the spore-bearing sporangium forms a whole layer of cells rather than being grouped into discrete piles called sori. There are about 6 genera and 200 species. The tree fern, Psaronius which is now extinct, was a member of this group.