Turning to Brongniart’s later work[595] we find an important proposal which led to no little controversy. While retaining the genus _Calamites_ for such specimens as possess a thin bark and a ribbed external surface, showing occasional branch-scars at the nodes, and having such characters as warrant their inclusion in the Equisetaceae, he proposes a second generic name for other specimens which had hitherto been included in _Calamites_. The fossils assigned to his new genus _Calamodendron_ are described as having a thick woody stem, and as differing from _Equisetum_ in their arborescent nature. Brongniart’s genus _Calamodendron_ is made to include the plants for which Cotta instituted the name _Calamitea_, and it is placed among the Gymnosperms. This distinction between the Vascular Cryptogam _Calamites_ and the supposed Gymnosperm _Calamodendron_ is based on the presence of secondary wood in the latter type of stem. The prominence formerly assigned to the power of secondary thickening possessed by a plant as a taxonomic feature, is now known to have been the result of imperfect knowledge. The occurrence of a cambium layer and the ability of a plant to increase in girth by the activity of a definite meristem, is a feature which some recent Vascular Cryptogams[596] share with the higher plants; and in former ages many of the Pteridophytes possessed this method of growth in a striking degree. Although Brongniart’s distinction between _Calamites_ and _Calamodendron_ has not been borne out by subsequent researches, the latter term is still used as a convenient designation for a special type of Calamitean structure. One of the earliest accounts of the anatomy of _Calamodendron_ stems is by Mougeot[597], who published figures and descriptions of two species, _Calamodendron striatum_ and _C. bistriatum_. Some years later Göppert[598], who was one of the greatest of the older palaeobotanists, instituted another genus, _Arthropitys_[599], for certain specimens of silicified stems from the Permian rocks of Chemnitz in Saxony, which Cotta had previously placed in his genus _Calamitea_ under the name of _Calamitea bistriata_[600]. Göppert rightly decided that the plants so named by Cotta differed in important histological characters from other species of _Calamitea_. The generic name _Arthropitys_ has been widely adopted for a type of Calamitean stem characterised by definite structural features. The great majority of the petrified Calamite stems found in the English Coal-Measures belong to Göppert’s _Arthropitys_. [Sidenote: WILLIAMSON.]