The radial longitudinal section represented in fig. 168, C, is taken from the fossil described by Weiss as a biseriate _Halonia_; it agrees sufficiently closely in structure with others referred to _Lepidodendron fuliginosum_ to be classed as an example of this anatomical type. A complete transverse section of the stem measures 9 × 6·3 cm.; the breadth of the tissues between the edge of the primary xylem and the outer edge of the secretory zone is 2·5 mm. The middle cortical region, characterised by the sooty appearance, which led Williamson to choose the specific name _fuliginosum_, is traversed by the leaf-traces and is sharply differentiated from both the inner and outer cortex. The longitudinal section (fig. 168, C) shows the outer edge of the primary xylem, _x_, abutting on a band of dark and small-celled parenchyma which passes into the broad zone of secondary tissue, _m_, the inner region of which consists of fairly thick-walled elements in radial series passing externally into the thin-walled cells of the cambial region, _a_, on the inner edge of the secretory zone, _sc_. This section shows also the interruption of the secretory zone by an outgoing leaf-trace, _lt_, the lower part of which, _sc_, is continued downwards into the secretory zone. The exit of a leaf-trace produces a gap in the secretory zone of the stem, but not in the xylem. If we applied the term phloem to the secretory zone—a course adopted by Prof. F. E. Weiss and some other authors, but which I do not propose to follow—we should speak of a phloem foliar-gap as a characteristic feature of a Lepidodendron shoot. This applies to other species of the genus as well as to _L. fuliginosum_. [Illustration: FIG. 171. _Lepidodendron fuliginosum._ (From sections in the Manchester Museum.)] Fig. 171, A, shows more clearly the broad zone of secondary parenchyma with the thinner-walled cambial region, _a_; the latter is represented on a larger scale in fig. 171, B. The section shown in fig. 168, D, and in fig. 170, A, affords an example of a stem in which the secondary tissue consists largely of narrow scalariform tracheae, _x_²; the primary stele has a diameter of 1 cm.; the secondary xylem, _x_², forms a fairly broad zone of parenchyma and tracheal elements through which leaf-traces pass vertically, a fact of some interest in comparison with the horizontal course which they pursue through the medullary rays in the normal secondary wood of _L. vasculare_ and _L. Wünschianum_. The secondary tracheae pass gradually into thin-walled cambial cells (_a_, fig. 168, D; 170, A) with parallel tangential walls. Fig. 171, C, shows the sinuous course of the secondary tracheae as seen in longitudinal section, and a few small groups of parenchymatous cells, _mr_, which may be of the nature of medullary rays, enclosed between the winding scalariform tracheae.