152 THE GEOLOGICAL HISTORY OF PLANTS. 



ceolate, sessile, entire, with rounded apices and of leathery con- 

 sistency. The leaves are from twenty to ninety centimetres in 

 length. The nerves are either equally or unequally strong. 



2. Dorycordaites. — Leaves lanceolate, with sharp points ; nerves 

 numerous, fine, and equal in strength. The leaves attain a length 

 of from forty to fifty centimetres. 



3. jPoacordaites. — Leaves narrow, linear, entire, blunt at the 

 point, with nerves nearly equally strong. The leaves are as much 

 as forty centimetres in length. 



To these Renault and Zeiller have added a fourth group, Seuto- 

 cordaites. 



Oenus Steenbergia. 



This is merely a provisional genus intended to receive casts of 

 the pith cylinders of various fossil trees. Their special peculiarity 

 is that, as in the modem Cecropia peltata, and some species of Ficus, 

 the pith consists of transverse dense partitions which, on the elonga- 

 tion of the internodes, become separated from each other, so as to 

 produce a chambered pith cavity, the cast of which shows transverse 

 furrows. The young twigs of the modern Abies halsamifera pre- 

 sent a similar structure on a minute scale. I have ascertained and 

 described such pith-cylinders in large stems of Dadoxylon Ouangon- 

 dianum, and O. materiarium. They occur also in the stems of 

 Cordaiies and probably of Sigillarim. I have discussed these curi- 

 ous fossils at length in "Acadian Geology" and in the "Journal of 

 the Geological Society of London," 1860. The following summary 

 is from the last-mentioned paper : 



a. As Prof. Williamson and the writer have shown, many of 

 the Stembergia piths belong to coniferous trees of the genus Da- 

 doxylon. 



b. A few specimens present multiporous tissue, of the type of 

 Dictyoxylon, a plant of unknown affinities, and which, according to 

 Williamson, has a Stembergia pith. 



c. Other examples show a, true scalariform tissue, comparable 

 with that of Lepidodendron or Sigillaria, but of finer texture. Corda 

 has shown that plants of the type of the former genus (his Loma- 

 tophloios) had Stembergia piths. Some plants of this group are by 

 external characters loosely reckoned by botanists as ribless Sigillarim 

 {Clathraria) ; but I believe that they are not related even ordinally 

 to that genus. 



d. Many Carboniferous Sternhergim show structures identical 

 with those described above as occurring in Gordaites, and also in 

 some of the trees ordinarily reckoned as Sigillarix. 



