To attempt a review of Williamson's work in fossil botany would be to write a treatise on the Carboniferous Flora. In every group--Calamites, Sphenophylls, Lycopods, Ferns, Pteridosperms, Gymnosperms--his researches are among the most important documents of the palaeobotanist, and to a great extent constitute the basis of our present knowledge. At the time he wrote, the wealth of his material was absolutely unrivalled, and its abundance was only equalled by the astonishing energy and skill with which he worked it out. As regards the Calamites, he demonstrated, to use his own words, "the unity of type existing among the British Calamites," abolishing the false distinction between Calamiteae and Calamodendreae. Among the Sphenophyllums (although there was at first some confusion in his nomenclature) he gave the first correct account of the anatomy, and of the organization of the cone. [Illustration: Plate XXIV Cone of _Calamostachys Binneyana_; sporangia and sporangiophores. _Drawn by Williamson_] Concerning the Lycopods, the greater part of our knowledge is due to him. He described the structure in ten species referred to _Lepidodendron_, besides other allied forms, and placed our knowledge of the comparative anatomy, once for all, on a broad and secure basis. His great monograph of _Stigmaria_, by some considered his best work, is still our chief authority for the subterranean organs. In the Ferns he made important contributions to our knowledge of the group now familiar to botanists as the Primofilices of Arber. In particular his account of the plant now known as _Ankyropteris corrugata_ is still among the best we possess of any member of the family. In Pteridosperms, to use the modern name, Williamson may fairly be called the discoverer of the important family Lyginodendreae. He appreciated their intermediate position, speaking of them, in 1887, as "possibly the generalised ancestors of both Ferns and Cycads." As regards both Pteridosperms and Gymnosperms proper, attention may be specially called to his work on isolated seeds, in which he was surpassed by Brongniart alone. This field of investigation, long neglected, has lately been revived with striking results. I hope that all students of fossil botany will have at least turned over the pages and the plates of Williamson's works, for only by inspection of the original memoirs can any idea be gained of his vast services to our science.