This specimen, which was figured by Williamson[626] in two of his papers, and by Lyell[627] in the fifth edition of his _Elementary Geology_, is historically interesting as being one of the first important plants obtained by Williamson early in the fifties, when he began his researches into the structure of Carboniferous plants. A joiner, who was employed by Williamson to make a piece of machinery for grinding fossils, brought a number of sandstone fragments as an offering to his employer, whom he found to be interested in stones. The specimens “were in the main the merest rubbish, but amongst them,” writes Williamson, “I detected a fragment which was equally elegant and remarkable.... In later days, when the specimen so oddly and accidentally obtained, came to be intelligently studied, its history became clear enough, and the priceless fragment is now one of the most precious gems in my cabinet[628].” _Comparison of three types of structure met with in Calamitean stems_,—Arthropitys, Arthrodendron, _and_ Calamodendron. [Sidenote: ARTHROPITYS.] The anatomical features which have so far been described as characteristic of _Calamites_ represent the common type met with in the English Coal-Measures. The same type occurs also in France, Germany and elsewhere. It is that form of stem known as _Arthropitys_, a sub-genus of _Calamites_. _Arthropitys_ may be briefly diagnosed as follows,—confining our attention to the structure of the stem: A ring of collateral bundles surrounds a large hollow pith, each primary xylem strand terminates internally in a more or less bluntly rounded apex traversed by a longitudinally carinal canal. The principal medullary rays consist of large-celled parenchyma, of which the individual elements are usually tangentially elongated as seen in transverse section, and four or five times longer than broad as seen in a tangential longitudinal section. The secondary xylem consists of scalariform and reticulately pitted tracheids; the interfascicular xylem may be formed completely across each primary ray at an early stage in the growth of the stem[629], or it may be developed more gradually so as to leave a tapering principal ray of parenchyma between each primary xylem bundle. In the latter case the principal rays present the characteristic appearance shown in figs. 71, 74, _A_, 75 and 78, a type of stem which we may refer to as _Calamites_ (_Arthropitys_) _communis_. In the former case the stem presents the appearance shown in fig. 83, _D_[630]. A third variety of _Arthropitys_ stem is one which was originally named by Göppert _Arthropitys bistriata_; in this form the principal rays retain their individuality as bands of parenchyma throughout the whole thickness of the wood[631]. Such stems as those of figs. 73 and 74, _B_, may be young examples of _Arthropitys communis_ or possibly of _A. bistriata_. The narrow secondary medullary rays of _Arthropitys_ usually consist of