Williamson appears to have been the first to recognise the presence, in the Carboniferous flora, of plants combining the characters of Ferns and Cycads. (See especially his "Organisation of the Fossil Plants of the Coal-Measures", Part XIII. "Phil. Trans. Royal Soc." 1887 B. page 299.) This conclusion was first reached in the case of the genera Heterangium and Lyginodendron, plants, which with a wholly fern-like habit, were found to unite an anatomical structure holding the balance between that of Ferns and Cycads, Heterangium inclining more to the former and Lyginodendron to the latter. Later researches placed Williamson's original suggestion on a firmer basis, and clearly proved the intermediate nature of these genera, and of a number of others, so far as their vegetative organs were concerned. This stage in our knowledge was marked by the institution of the class Cycadofilices by Potonie in 1897. Nothing, however, was known of the organs of reproduction of the Cycadofilices, until F.W. Oliver, in 1903, identified a fossil seed, Lagenostoma Lomaxi, as belonging to Lyginodendron, the identification depending, in the first instance, on the recognition of an identical form of gland, of very characteristic structure, on the vegetative organs of Lyginodendron and on the cupule enveloping the seed. This evidence was supported by the discovery of a close anatomical agreement in other respects, as well as by constant association between the seed and the plant. (F.W. Oliver and D.H. Scott, "On the Structure of the Palaeozoic Seed, Lagenostoma Lomaxi, etc." "Phil. Trans. Royal Soc." Vol. 197 B. 1904.) The structure of the seed of Lyginodendron, proved to be of the same general type as that of the Cycads, as shown especially by the presence of a pollen-chamber or special cavity for the reception of the pollen-grains, an organ only known in the Cycads and Ginkgo among recent plants.