No one has done more than my friend Dr. Williamson, of Manchester, to illustrate the structure of Calamites, and he has shown that these plants, like other cryptogams of the Carboniferous, had mostly stems with regular fibrous wedges, like those of exogens. The structure of the stem is, indeed, so complex, and differs so much in different stages of growth, and different states of preservation, that we are in danger of falling into the greatest confusion in classifying these plants. Sometimes what we call a Calamite is a mere cast of its pith showing longitudinal striæ and constrictions at the nodes. Sometimes we have the form of the outer surface of the woody cylinder, showing longitudinal ribs, nodes, and marks of the emission of the branchlets. Sometimes we have the outer surface of the plant covered with a smooth bark showing flat ribs, or almost smooth, and having at the nodes regular articulations with the bases of the verticillate branchlets, or on the lower part of the stem the marks of the attachment of the roots. The Calamites grew in dense clumps, budding off from one another, sometimes at different levels, as the mud or sand accumulated about their stems, and in some species there were creeping rhizomata or root-stocks (Figs. 46 to 49). [Illustration: Fig. 46.--_Calamites_. A, _C. Suckovii_. B, _C. Cistii_. (From "Acadian Geology.")] [Illustration: Fig. 47.--Erect _Calamites_, with roots attached (Nova Scotia).] [Illustration: Fig. 48.--Node of _C. Cistii_, with long leaves (Nova Scotia).] But all Calamites were not alike in structure. In a recent paper[CC] Dr. Williamson describes three distinct structural types. What he regards as typical Calamites has in its woody zone wedges of barred vessels, with thick bands of cellular tissue separating them. A second type, which he refers to _Calamopitus_, has woody bundles composed of reticulated or multiporous fibres, with their porous sides parallel to the medullary rays, which are better developed than in the previous form. The intervening cellular masses are composed of elongated cells. This is a decided advance in structure, and is of the type of those forms having the most woody and largest stems, which Brongniart named _Calamodendron_ (Fig. 50). A third form, to which Dr. Williamson seems to prefer to assign this last name, has the tissue of the woody wedges barred, as in the first, but the medullary rays are better developed than in the second. In this third form the intermediate tissue, or primary medullary rays, is truly fibrous, and with secondary medullary rays traversing it. My own observations lead me to infer that there was a fourth type of calamitean stem, less endowed with woody matter, and having a larger fistulous or cellular cavity than any of those described by Dr. Williamson. [CC] "Memoirs of the Philosophical Society," Manchester, 1886-'87.