The trunks of this genus occur from the Middle Devonian to the Permian inclusive, as drift-logs calcified, silicified, or pyritised. The only foliage associated with them is of the type of _Walchia_ and _Araucarites_--viz., slender branches with numerous small spiral acicular leaves. Two of the coal-formation species, _D. materiarum_ and another, had foliage of this type. That of the others is unknown. They are all distinct from the wood of _Cordaites_, for which see under that genus. The following are North American species: _Trunks._ _Dadoxylon Ouangondianum_, Dn. M. Erian Report, 1871.[CP] _D. Halli_, Dn. " " _D. Newberryi_, Dn. " " _D. Clarkii_, Dn. (Cordæoxylon ?) " Report, 1882. _D. Acadianum_, Dn. Coal-formation Acadian Geology. and millstone grit. _D. Materiarum_, Dn. Do. and Permo- " Carb. _D._ (_Palæoxylon_) _antiquius_, Dn. L. Carboniferous. " _D. annulatum_, Dn. Coal-formation. " _Ormoxylon Erianum_, Dn. Erian Report, 1871. _Foliage._ _Araucarites gracilis_, Dn. N. Coal-formation " and Permian. { Report on _Walchia robusta_, Dn. Permian. { Prince Edward _W. imbricatula_, Dn. " { Island. [CP] "Geological Survey of Canada: Fossil Plants of Erian and Upper Silurian Formations," by J. W. Dawson. All of the above can be vouched for as good species based upon microscopic examination of a very large number of trunks from different parts of North America. The three Erian species of _Dadoxylon_ and _D. antiquius_ from the Lower Carboniferous have two or more rows of cells in the medullary rays. The last named has several rows, and is a true _Palæoxylon_ allied to _D. Withami_ of Great Britain. _D. materiarium_ is specially characteristic of the upper coal-formation and Permian, and to it must belong one or both of the species of foliage indicated above. _D. Clarkii_ has very short, simple medullary rays of only a few cells superimposed, and has an inner cylinder of scalariform vessels, approaching in these points to _Cordaites_. _Ormoxylon_ has a very peculiar articulated pith and simple medullary rays.