The origin of the leaf-traces from the middle of the sinuses on the edge of the primary xylem is regarded as a difference; in _Lepidodendron_ the leaf-traces are said to arise in some species from the sides of the crenulations; but, as already pointed out, this is a distinction of doubtful value. The division of the primary xylem into separate strands in some stems of _Sigillaria_ of the Clathrarian and Leiodermarian forms is a characteristic peculiarity; but _S. spinulosa_ forms a connecting link between this type and the continuous arrangement of the xylem in _S. elongata_ and _S. elegans_. Kidston[536] has shown that the discontinuous primary xylem occurs in Lower Permian species, a fact consistent with the view that the greater abundance of the centripetally developed wood, characteristic of the older species, represents a more primitive feature. This is not merely a conclusion drawn from a consideration of geological age, but it is in harmony with the view expressed by Scott[537] that as plants achieved greater success in producing secondary centrifugal wood, the retention of any considerable quantity of primary xylem became superfluous. As yet we know very little of the structure of the perixylic tissues of _Sigillaria_, but there is no sufficient reason for supposing that these differ in essentials from those in _Lepidodendron_. The middle and outer cortical tissues are practically identical in the two genera. The parichnos is of the same type, except that in _Sigillaria_ it reached greater dimensions in the outer part of its course. V. _Sigillaria Brardi_[538] Brongniart. Figs. 196, A–C; 200; 203. 1822. _Clathraria Brardi_, Brongniart, Classif. Vég. foss., Pl. XII. fig. 5. 1828. _Sigillaria Brardi_, Brongniart, Hist. Vég. foss. p. 430, Pl. CLVIII. fig. 4. _S. Menardi_, _ibid._ Pl. CLVIII. 1836. _Lepidodendron Ottonis_, Goeppert, Fossil Farnkr. Pl. XLII. 1839. _S. elegans_, Brongniart, Arch. Mus. Nat. Hist. Paris, Vol. I. p. 406, Pl. XXV. 1849. _S. spinulosa_, Germar, Verstein. Wettin und Löbejün, p. 59, Pl. XXV. 1893. _S. mutans_, Weiss, Abhand. Preuss. Geol. Anst. [N.F.] Heft 2, Pl. VIII. [Illustration: FIG. 203. _Sigillaria Brardi_ Brongn. (¾ nat. size). From a photograph of a specimen in Dr Kidston’s collection, from the Upper Transition Series of Staffordshire. Published by Kidston (02) Pl. LIX. fig. 1.]