species, and has probably been responsible for erroneous specific diagnoses. In some recent Horse-tails the sheath is occasionally divided in one or two places by a slit reaching to the base of the coherent segments[498]; this shows a tendency of the segments towards the free manner of occurrence which is usually considered a Calamitean character. In certain fossils referred to the genus _Annularia_, the nodes bear whorls of long and narrow leaves which are fused basally into a collar (fig. 58, _D_). There are good grounds for believing that at least some Annularias were the foliage shoots of true Calamites. Again, in some species of _Calamitina_, a sub-genus of _Calamites_, the leaves appear to have been united basally into a narrow sheath. We see, then, that it is a mistake to attach great importance to the separate or coherent character of leaf-segments in attempting to draw a line between the true _Calamites_ and _Equisetites_. Potonié[499] while pointing out that this distinction does not possess much value as a generic character, retains the genus _Equisetites_ for certain Palaeozoic Equisetum-like leaf-sheaths. [Illustration: FIG. 56. Calamitean leaf-sheath. From a specimen in the Woodwardian Museum. _a_, base of leaf-sheath; (very slightly reduced).] Fig. 56 represents a rather faint impression of a leaf-sheath and nodal diaphragm. The specimen is from the Coal-Measures of Ardwick, Manchester. The letter _a_ probably points to the attachment of the sheath to the node of the stem. The flattened sheath is indistinctly divided into segments, and at the middle of the free margin there appears to be a single free tooth. The lower part of the specimen, as seen in the figure, shows the position of the nodal diaphragm. Between the diaphragm and the sheath there are several slight ridges converging towards the nodal line; these agree with the characteristic ridges and grooves of Calamite casts which are described in detail in Chapter X. There is another specimen in the British Museum which illustrates, rather more clearly than that shown in fig. 56, the association of a fused leaf-sheath with a type of cast usually regarded as belonging to a Calamitean stem. Some leaf-sheaths of Permian age described by Zeiller[500] as _Equisetites Vaujolyi_ bear a close resemblance to the sheath in fig. 58 E. The nature of the true Calamite leaves is considered more fully on a later page. [Sidenote: PALAEOZOIC EQUISETITES.]