The genus _Lyginopteris_ is selected for the first place in this chapter simply on the ground that we have a fuller knowledge of its morphology than in the case of other types. It is not regarded as the most primitive member of its class. _Lyginopteris_ may be described in a few words as a plant having the habit and to a large extent the anatomical features of a Fern, but differing from existing ferns in the possession of integumented megasporangia or seeds and in the power of secondary growth in thickness by means of a cambium in both stem and root. The seed (_Lagenostoma_) agrees with those of recent Cycads and Gnetales more closely than with the corresponding organs in Conifers or any other group, while the structure of the secondary wood is practically identical with that of Cycads. The microsporangia occur as groups of small bilocular sporangia, or synangia, at the tips of fertile pinnae of highly compound fronds. _Nomenclature and Historical Summary._ In 1866 E. W. Binney[93] of Manchester published a short description of a small petrified stem from the Lower Coal Measures of Lancashire and named it _Dadoxylon oldhamium_, employing Endlicher’s term _Dadoxylon_ which that author substituted for _Pinites_ as previously used by Witham[94]. Three years later Williamson[95] drew attention to certain features in which Binney’s type differs from the genus _Dadoxylon_ and substituted a new name _Dictyoxylon_, suggested by the reticulate pitting on the walls of the tracheids. In a subsequent paper Williamson[96] gave a fuller description of Binney’s species and spoke of it as ‘one of the most common plants in the calcareous nodules of the Lower Coal Measures’ of Lancashire and Yorkshire. He connected certain casts of arborescent dimensions with Binney’s type on the ground that the surface-features of the casts are such as would be produced by partially decorticated stems having a hypodermal reticulum of mechanical tissue like that preserved in the small petrified specimen described by Binney (fig. 402). Mr Carruthers called Williamson’s attention to a paper by Mr Gourlie[97] in which the generic name _Lyginodendron_ is instituted for stem-casts identical in surface-features with the fossils figured by Williamson. In spite of the much larger dimensions of the reticulum on the casts described by Gourlie as compared with that in the outer cortex of Binney’s stem, Williamson concluded that _Lyginodendron_ is ‘undoubtedly an inorganic cast of the prosenchymatous layer of the bark of _Dictyoxylon_.’ It is but fair to add that Williamson was influenced in coming to this conclusion by a discovery by Mr Nield of a piece of a large petrified stem believed to be generically identical with Binney’s type, but subsequently referred to a distinct genus[98], which was comparable in size with the stems responsible for Gourlie’s _Lyginodendron_ casts. The type-specimen of Gourlie’s _Lyginodendron