From the Upper Devonian to the Permian epoch _Calamites_ and other closely related types played a prominent part in the vegetation of the world. We have no good evidence for the existence of _Calamites_ in Triassic times; in its place there were gigantic Equisetums which resembled modern Horse-tails in a remarkable degree. In the succeeding Jurassic period tree-like Equisetums were still in existence, and species of _Equisetites_ are met with in rocks of this age in nearly all parts of the world. A few widely distributed species are known from Wealden rocks, but as we ascend the geologic series from the Jurassic strata, the Equisetums become less numerous and the individual plants gradually assume proportions practically identical with those of existing forms. A. _Equisetites._ The generic name _Equisetites_ was proposed by Sternberg in 1838[496] as a convenient designation for fossil stems bearing a close resemblance to recent species of _Equisetum_. Some authors have preferred to apply the name _Equisetum_ to fossil and recent species alike, but in spite of the apparent identity in the external characters of the fossil stems with those of existing Horse-tails, and a close similarity as regards the cones, there are certain reasons for retaining Sternberg’s generic name. It is important to avoid such nomenclature as might appear to express more than the facts admit. If the custom of adding the termination -_ites_ to the root of a recent generic term is generally followed, it at once serves to show that the plants so named are fossil and not recent species. Moreover, in the case of fossil Equisetums we know nothing of their internal structure, and our comparisons are limited to external characters. Stems, cones, tubers, and leaves are often very well preserved as sandstone casts with distinct surface-markings, but we are still in want of petrified specimens. There is indeed evidence that some of the Triassic and Jurassic species of _Equisetites_, like the older Calamites, possessed the power of secondary growth in thickness, but our deductions are based solely on external characters.