In the meantime he had returned to the Yorkshire Oolitic plants and, about 1847, published a paper in the Proceedings of the Yorkshire Philosophical Society, "On the Scaly Vegetable Heads or collars from Runswick Bay, supposed to belong to the _Zamia gigas_." His full paper, in which he maintained the Cycadean affinities of the flower-like fossils, was written soon afterwards, but met with a series of misfortunes, and was not finally published till 1870, in the _Transactions of the Linnean Society_, before which body it had been read in 1868. Williamson was admittedly right in connecting the floral organs with the so-called _Zamia_ foliage, and his interpretation of the complicated structure was as good as was possible in the then state of knowledge. The true nature of these fossils, now known by the name _Williamsonia_, given them by Mr Carruthers, could only be understood at a much later date in the light of Dr Wieland's famous researches on the American Bennettiteae, and has quite recently been made clear in a memoir by Prof. Nathorst. Perhaps, even now, some points remain doubtful. Early in the fifties Williamson made some rough sections of a Calamite which came into his hands, and this was the beginning of his most characteristic line of work. A remarkable internal cast of a Calamite, figured by Lyell in his _Manual of Geology_ in 1855, led to a correspondence with M. Grand'Eury, now so famous as the veteran French palaeobotanist. Williamson at that time had no intention of entering on the serious study of Carboniferous plants, for Binney was already in the field. Grand'Eury's letter, however, caused him to look up his old sections, which he found differed from the Calamitean stems figured by Binney. Matters for a time moved slowly, and Williamson's specimen was only described in 1868 in the _Manchester Memoirs_. This fossil, which he named _Calamopitus_, is now known as _Arthrodendron_, and is a distinct type of Calamarian stem, intermediate between the common _Calamites_ or _Arthropitys_, and the more elaborate _Calamodendron_ of the Upper Coal Measures. Williamson was now fairly started on his Carboniferous work. His first memoir on the Organisation of the Fossil Plants of the Coal Measures was communicated to the Royal Society on November 11, 1870. It is amusing to find that the secretaries objected to the memoir being called Part I, since it bound the society to publish a Part II! Nineteen Parts were published, the last in 1893.