This order was founded by Unger[507] for some imperfectly preserved stems from Palaeozoic strata in Thuringia and in it he included the two genera _Cladoxylon_ and _Schizoxylon_. There is some doubt as to the precise age of the Thuringian beds; they were assigned by Richter to the Devonian system and subsequently placed in the Culm: Solms-Laubach in his later reference to Unger’s plants favours a Devonian horizon[508]. Unger included the Cladoxyleae in the Lycopodiales, and though this conclusion is not accepted the position of the order is still uncertain. His genus _Schizoxylon_ has no claim to generic separation from _Cladoxylon_. An inspection of the illustrations in the memoir by Richter and Unger reveals a striking resemblance in the main anatomical features between several types assigned to different genera and distributed among the Cladoxyleae and Rhachiopterideae (a term first used by Corda for petrified rachises or petioles of ferns) and other orders. Solms-Laubach[509], to whom our more accurate information as to Unger’s plants is chiefly due, is inclined to regard the specimens referred by Unger to the genus _Arctopodium_ as young stems of _Cladoxylon_, and he draws attention to a close similarity between _Hierogramma_, another of Unger’s genera, and _Cladoxylon_. Paul Bertrand[510] goes further in considering that the following genera represent one generic type, namely _Syncardia_ (fig. 459, F), _Hierogramma_, _Arctopodium_, _Cladoxylon_, and _Schizoxylon_. The same author interprets the fossils so named by Unger as stems and does not agree with the inclusion of any of them in the Rhachiopterideae. Without losing sight of the fact that Bertrand’s conclusion is not based on proof but is the expression of a view suggested by a close agreement in general anatomical plan, I venture to adopt the designation _Cladoxylon_ in a wide sense primarily on the ground that Bertrand’s view is probably correct and in part for the sake of convenience of description. As Unger’s species of _Cladoxylon_ differ from one another in features which may fairly be regarded as of minor importance, they are included under one specific name. =Cladoxylon=. Unger. _Cladoxylon mirabile_ Unger[511]. The following are regarded as specifically identical with or closely allied to _Cladoxylon mirabile_: _C. dubium_, _Schizoxylon taeniatum_, _Hierogramma mysticum_, _Syncardia pusilla_, _Arctopodium insigne_ and _A. radiatum_[512].