Primitive pteridophytes, which were the earliest vascular plants, from the Silurian and the Devonian. They had slender, tapering, leafless or scale-bearing stems up to 50 cm high, often with cone-shaped sporangia (see spore) at the top. Some authorities placed the fossil groups with the present-day psilophytes (Psilotum, Tmesipteris) in the class Psilopsida, comprising the orders Psilotales (living forms) and Psilophytales (fossil forms). In more recent classifications the members of the Psilophytales have been placed into three separate groups, usually ranked as subdivisions: Rhyniophytina (e.g. Cooksonia and Rhynia); Trimerophytina (e.g. Psilophyton and Trimerophyton); and Zosterophyllophytina (e.g. Zosterophyllum). The living forms are contained in a fourth subdivision, the Psilophytina.