In the works of earlier writers we find frequent illustrations of specimens of _Annularia_, which are compared with Asters and other recent flowering plants. Lehmann[658] contributed a paper to the Royal Academy of Berlin in 1756, in which he referred to certain fossil plants as probable examples of flowers, among them being a specimen of _Annularia_. He refers to the occurrence of fossil ferns and other plants, and asks why we do not find flowers of the rose or tulip; his object being “not to acquire vain glory, but to give occasion for others to look into the matter more clearly.” The general habit of the fossils which are now included under _Annularia_ agrees closely with that of _Calamocladus_. There is the same spreading form and a similar foliage in the two genera, but in _Annularia_ the members of a whorl are always fused into a basal sheath, and the segments are not of equal length. We may thus summarise the characteristic features of the genus:— Opposite branches are given off in one plane from the nodes of a main axis; the leaves are in the form of narrow sheaths divided into numerous and unequal linear or narrow lanceolate segments, each with a median vein. The segments in each whorl appear to be spread out in one plane very oblique to the axis of a branch, instead of spreading radially in all directions; the lateral segments are usually longer than the upper and lower members of a whorl. The vegetative branches possess the same type of structure as _Calamites_. A comparison of _Annularia_ and _Phyllotheca_ has already been made in Chapter IX. (p. 282). Potonié[659] has recently given a detailed account of Annularian leaves; he compares them with those of _Equisetum_, and describes the occurrence on the lamina of each leaf-segment of a broad central band or midrib, with a groove, probably containing stomata, on either side. He shows that in well-preserved specimens of _Annularia_, it is possible to recognise certain minute surface-features, such as the presence of hairs and stomata, which enable one to detect a close resemblance between the leaves of Calamite stems and those of Annularian shoots. It is not always easy to distinguish between _Annularia_ and _Calamocladus_; the collar-like basal sheath in the leaves of the former is a characteristic feature, but that cannot always be recognised. On the other hand, the leaves of _Calamocladus_ may sometimes be flattened out on the surface of the rock and simulate the deeply cut sheaths of _Annularia_. It is difficult to decide how far the manner of occurrence of Annularian leaves in one plane, which is commonly insisted on as a generic character, is an original feature, or how far it is the result of compression in fossilisation. Probably the leaves of a living _Annularia_ were spread out at right angles to the axis, as in the ‘verticils’ of such a plant as _Galium_.