XVI] SIGILLARIA 221 



produce depressions on the exposed face of the dead stem. 

 Casts of this type are not uncommon in Carboniferous rocks, 

 and while some may belong to the Pteridosperm Lyginodendron, 

 others may be those of Sigillarian stems. 



The large parichnos-strands, produced as in Lepidodendron, 

 by the forking of a single strand arising in the middle cortical 

 region, consist in part of tissue containing secretory canals, a 

 structure like that recently described by Miss Coward^ in the 

 large parichnos strands of Syringodendron stems. 



An example of a decorticated specimen is described by 

 Renault^ as Sigillaria xylina. This stem is presumably referred 

 to Sigillaria because the primary xylem consists of separate 

 strands. It is characterised by the unusually large develop- 

 ment of secondary wood and by the relatively small size of the 

 pith. The xylem cylinder has a diameter of 4 — 5 cm. and the 

 pith is only 4 — 5 mm. in breadth. 



Another example of a petrified Sigillaria stem has been 

 described by Kidston^ as S. elegans Brongn.^ (fig. 193, D), a 

 species characterised by vertical rows of sub-hexagonal and 

 contiguous leaf-scars and by the presence of verticils of cone- 

 scars. Fig. 193, D, represents Kidston's specimen in surface- 

 view ; one row of leaf-scars is shown, but most of the superficial 

 tissues have been destroyed. The crushed stele, 13 mm. in its 

 longest diameter, has a continuous cylinder of primary xylem, 

 (fig. 202, A, x) characterised by a regularly crenulate outer 

 margin with the smallest elements at the edge ; the prominent 

 ridges separating the sinuses are rounded. The leaf-traces arise 

 from the bottom of each sinus; the leaf-bundles are mesarch, and 

 consist exclusively of primary elements. The secondary xylem, 

 x^, like that of the primary xylem, has a crenulate outer edge. 

 The most interesting feature of the outer cortex is afforded by 

 a tangential section which, in addition to the leaf-scars, cuts 

 through a cone-scar showing a solid primary stele surrounded 

 by the cortex of the cone-peduncle. 



Another type of Sigillaria, probably S. elongata Brongn. 

 (fig. 202, B, C), which is very similar to S. scidellaia has been 



1 Coward (07). ^ Eenault (96) A. p. 237, PI. xxxviii. figs. 1-4. 



■' Kidston (05). * Brongniart (28) A. Pis. cxlvi. clv. clviii. 



