The next proposal to be noticed is one by Williamson[601] in 1868; he instituted the generic name _Calamopitys_ for a few examples of English stems, which differed in the structure of the wood and primary medullary rays from previously recorded types. We have thus four names which all stand for generic types of Calamitean stems. Of these _Calamodendron_ and _Arthropitys_ are still used as convenient designations for stems with well-defined anatomical characters. The genus _Calamitea_ is no longer in use, and Williamson’s name _Calamopitys_ had previously been made use of by Unger[602] for plants which do not belong to the Calamarieae. As it is convenient to have some term to apply to such stems as those which Williamson made the type of _Calamopitys_, the name _Arthrodendron_ is suggested by my friend Dr Scott[603] as a substitute for Williamson’s genus. The twofold division of the Calamites instituted by Brongniart has already been alluded to, and for many years it was generally agreed that both Pteridophytes and Gymnosperms were represented among the Palaeozoic fossils known as Calamites. The work of Prof. Williamson was largely instrumental in proving the unsound basis for this artificial separation; he insisted on the inclusion of all Calamites in the Vascular Cryptogams, irrespective of the presence or absence of secondary wood. By degrees the adherents of Brongniart’s views acknowledged the force of the English botanist’s contention. It is one of the many signs of the value of Williamson’s work that there is now almost complete accord among palaeobotanical writers as to the affinities of Calamitean plants. In the following account of the Calamites, the generic name _Calamites_ is used in a wide sense as including stems possessing different types of internal structure; when it is possible to recognise any of these structural types the terms _Calamodendron_, _Arthropitys_ or _Arthrodendron_ are used as subgenera. The reasons for this nomenclature are discussed in a later part of the Chapter. ╭ This term was originally applied Genus _Calamites_, Suckow, 1714 ┤ to the common pith-casts of │ Calamitean stems, _without reference ╰ to_ _internal structure_. Subgenera _Calamodendron_, Brongniart, 1849 ╮ These names have _Arthropitys_ Göppert, 1864 ├ primarily reference _Arthrodendron_ Scott 1897 │ to _internal_ (= _Calamopitys_ Williamson, 1871) ╯ _structure_. II. Description of the anatomy of Calamites. _a. Stems. b. Leaves. c. Roots. d. Cones._