A group long recognized as an order of fossil plants (subdivision Psilopsida) collected in rocks of Late Silurian and Devonian age. It has been subdivided, on the basis of many data, into three categories whose descriptions include the chief kinds of plants formerly included in Psilophytales. Some authors will treat the three groups as classes under the division Rhyniophyta; others may prefer to consider them as three separate divisions of the subkingdom Embryobionta. They are given in this Encyclopedia as three classes of the division Rhyniophyta: Rhyniopsida, Zosterophyllopsida, and Trimerophytopsida.