The generic or sub-generic title Rhytidolepis, also instituted by Sternberg, is applied to ribbed Sigillarian stems such as S. scutellata, S. rugosa ([fig. 193], B), S. mammillaris ([fig. 195]), or S. laevigata ([fig. 196], D). Goldenberg[456] proposed the name Leiodermaria for smooth Sigillarian stems with leaf-scars not in contact with one another ([fig. 196], C).

The shoot system of Sigillaria consisted of a stout stem tapering upwards to a height of 100 feet[457] or more as an unbranched column, with its dome-shaped apex[458] covered with linear grass-like leaves or, in some species, such as Sigillaria Brardi[459], S. Eugenii[460], etc., the main trunk was occasionally divided by apparently equal dichotomy. The younger portions of the stem or branches were in some species clothed with leaves separated by a narrow zigzag groove surrounding their hexagonal bases, while in other forms each leaf was seated on a more or less prominent cushion having the form illustrated by Sigillaria McMurtriei ([fig. 194]) or by the example represented in [fig. 200], H; or as in the ribbed species shown in figs. [193], B, and [195], the leaves in vertical series were separated from one another by longer portions of the ribs. As in Lepidodendron the cushions are frequently characterised by irregular transverse wrinklings and other[461] surface-ornamentation which in some instances at least may have been produced as the result of post-mortem shrinkage of superficial tissue. From the rarity of shoots with the foliage attached, it would seem that the leaves persisted for a comparatively short time and were cut off by an absciss-layer leaving behind a well-marked leaf-scar area. The linear leaves, reaching in rare cases a length of one metre (e.g. S. lepidodendrifolia) but usually much shorter, possessed a single median bundle, and the lower face was characterised by two stomatal grooves and a median keel. It is not uncommon to find leaf-bases of Sigillaria detached from the stem and preserved as separate impressions. The term Sigillariophyllum used by Grand’Eury[462] may be applied to detached leaves, though it is by no means easy to distinguish between the foliage of Sigillaria and Lepidodendron. A comparison of a typical species of Sigillaria, such as S. rugosa ([fig. 193], B) or S. Brardi ([fig. 196], A–C) with a typical Lepidodendron reveals obvious differences in the form of the leaf-cushion, but in some cases the distinction becomes purely arbitrary.

Fig. 194. Sigillaria McMurtriei Kidst. From a specimen from the Upper Coal-Measures of Radstock, in the British Museum (V. 952). Nat. size.

Fig. 195. Sigillaria mammillaris. (Rhytidolepis form.) From a specimen in the Manchester Museum. p, parichnos; l, ligule-pit; t, leaf-trace; c, cushion; s, leaf-scar.

Fig. 196.