The specimens of fossil Equisetums are usually readily recognised by the coherent leaf-segments in the form of nodal sheaths resembling those of recent species. The tissues of the cortex and central cylinder are occasionally represented by a thin layer of coal pressed on to the surface of a sandstone cast, or covering a flattened stem-impression on a piece of shale. It is sometimes possible under the microscope to recognise on the carbonised epidermal tissues the remains of a surface-ornamentation similar to that in recent species, which is due to the occurrence of siliceous patches on the superficial cells. Longitudinal rows of stomata may also be detected under favourable conditions of preservation. The nodal diaphragms of stems have occasionally been preserved apart, but such circular and radially-striated bodies may be misleading if found as isolated objects. Casts of the wide hollow pith of _Equisetites_, with longitudinal ridges and grooves, and fairly deep nodal constrictions, have often been mistaken for the medullary casts of _Calamites_. Several species of _Equisetites_ have been recorded from the Upper Coal-Measures and overlying Permian rocks, but these present special difficulties. In one instance described below, (_Equisetites Hemingwayi_ Kidst.), the species was founded on a cast of what appeared to be a strobilus made up of sporophylls similar to those in an _Equisetum_ cone. In other Permo-Carboniferous species the choice of the generic name _Equisetites_ has been determined by the occurrence of leaf-sheaths either isolated or attached to the node of a stem. The question to consider is, how far may the Equisetum-like leaf-sheath be regarded as a characteristic feature of _Equisetites_ as distinct from _Calamites_? In the genus _Calamites_ the leaves are generally described as simple linear leaves arranged in a whorl at the nodes, but not coherent in the form of a sheath (fig. 85). The fusion of the segments into a continuous sheath or collar is regarded as a distinguishing characteristic of _Equisetites_ and _Equisetum_. The typical leaf-sheath of a recent Horse-tail has already been described. In some species we have fairly large and persistent free teeth on the upper margin of the leaf-sheath, but in other Equisetums the rim of the sheath is practically straight and has a truncated appearance, the distal ends of the segments being separated from one another by very slight depressions, as in a portion of the sheath of _Equisetum ramosissimum_ Desf. of fig. 58, _C_. In other leaf-sheaths of this species there are delicate and pointed teeth adherent to the margin of the coherent segments; the teeth are deciduous, and after they have fallen the sheath presents a truncated appearance. This difference between the sheaths to which the teeth are still attached and those from which they have fallen is illustrated by fig. 58, _B_ and _C_; it is one which should be borne in mind in the description of fossil