The Stigmarian axes and their rootlets are very commonly found in the “underclays” and “gannister” beds which lie below the coal seams (see p. 25), and they may sometimes be seen attached to a bit of the trunk growing upwards through the layers. They and the aerial stems of Lepidodendron are perhaps the commonest and most widely known of fossil plants. Before leaving the palæozoic Lycopods another genus must be mentioned, which is also a widely spread and important one, though it is less well known than its contemporary. The genus _Sigillaria_ is best known by its impressions and casts of stems covered by leaf scars. The stems were sometimes deeply ribbed, and the leaf scars were arranged in rows and were more or less hexagonal in outline, as is seen in fig. 102, which shows a cast and its reverse of the stem of a typical _Sigillaria_. [Illustration: Fig. 102.—Cast and Reverse of Leaf Scars of _Sigillaria_. In A the shape of the leaf bases is clearly shown, the central markings in each being the scar of the vascular bundle and parichnos] In its primary wood _Sigillaria_ differed from _Lepidodendron_ in being more remote from the type with a primary solid stele. Its woody structure was that of a ring, in some cases irregularly broken up into crescent-shaped bundles. The secondary wood was quite similar to that of _Lepidodendron_. _Stigmaria_ and its rootlets belong equally to the two plants, and hitherto it has been impossible to tell whether any given specimen of _Stigmaria_ had belonged to a _Lepidodendron_ or a _Sigillaria_. Between the two genera there certainly existed the closest affinity and similarity in general appearance. These two genera represent the climax of development of the Lycopod family. In the Lower Mesozoic some large forms are still found, but all through the Mesozoic periods the group dwindled, and in the Tertiary little is known of it, and it seems to have taken the retiring position it occupies to-day. CHAPTER XV PAST HISTORIES OF PLANT FAMILIES VIII. The Horsetails