THE CARBONIFEROUS FLORA. 



159 



5. Leioderma, Goldenberg. Type, S. Sydnensis, Dawson. — 

 Ribs obsolete. Cortical and ligneous surfaces striate. Vascular 

 scars double, elongate longitudinally, and alike on cortical and inner 

 surfaces. Areoles in rows and distinct ; stigmaria-roots striate, with 

 small and distinct areoles. 



6. Clathraria, Brongniart. Type, S. Menardi, Brongniart. — 

 Areoles hexagonal, not in distinct rows, but having a spiral appear- 

 ance. Some of the plants usually referred to this group are probably 

 branches of Favularia. Others are evidently fragments of plants 

 of the genus Lepidophloios. 



3. Internal Structures of Sigillaria- Stems. 



I long ago pointed out, on the evidence of the external markings 

 and mode of growth, that the stems of SigiUarics must have been 

 exogenous, and this conclusion has now been fully confirmed by the 

 microscopic researches of Williamson, not only in the case of Sigil- 

 larice, but of Lepidodendra and Calamodendra as well. Confining 

 myself to my own observations, three types of Sigillarim are known 

 to me by their internal structures, though I cannot certainly corre- 

 late all of these with the external markings referred to above. 



1. Diploxylon, in which the stem consists of a small internal 

 axis surrounded by a very thick inner bark and a dense outer cortex. 

 A fine example from the South Joggins is thus described : * 



" The axis of the stem is about six centimetres in its greatest 

 diameter, and consists of a central pith-cylinder and two concen- 

 tric coats of scalariform tissue. The pith-cylinder is replaced 

 by sandstone, and is about one centimetre in diameter. The inner 

 cylinder of scalariform tissue is perfectly continuous, not radiated, 

 and about one millimetre in thickness. Its vessels are somewhat 

 crushed, but have been of large diameter. Its outer surface, which 

 readily separates from that of the outer cylinder, is striated longi- 

 tudinally. The outer cylinder, which constitutes by much the 

 largest part of the whole, is also composed of scalariform tissue; 

 but this is radially arranged, with the individual cells quadrangular 

 in cross-section. The cross-bars are similar on all the sides and 

 usually simple and straight, but sometimes branching or slightly 

 reticulated. The wall intervening between the bars has extremely 

 delicate longitudinal waving lines of ligneous lining, in the manner 

 first described by Williamson as occurring in the scalariform tissue 

 of certain Lepidodendra. A few small radiating spaces, partially 



" Journal of the Geological Society of London," November, 1877. 



