To these Renault and Zeiller have added a fourth group, Scutocordaites.
Genus Sternbergia.
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 modern Cecropia peltata, and some species of Ficus, the pith consists of transverse dense partitions which, on the elongation 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 balsamifera present a similar structure on a minute scale. I have ascertained and described such pith-cylinders in large stems of Dadoxylon Ouangondianum, and D. materiarium. They occur also in the stems of Cordaites and probably of Sigillariæ. I have discussed these curious 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 Sternbergia piths belong to coniferous trees of the genus Dadoxylon.
b. A few specimens present multiporous tissue, of the type of Dictyoxylon, a plant of unknown affinities, and which, according to Williamson, has a Sternbergia 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 Lomatophloios) had Sternbergia piths. Some plants of this group are by external characters loosely reckoned by botanists as ribless Sigillariæ (Clathraria); but I believe that they are not related even ordinally to that genus.
d. Many Carboniferous Sternbergiæ show structures identical with those described above as occurring in Cordaites, and also in some of the trees ordinarily reckoned as Sigillariæ.
Genus Cardiocarpum.
I have found at least eight species of these fruits in the Erian and Carboniferous of New Brunswick and Nova Scotia, all of which are evidently fruits of gymnospermous trees. They agree in having a dense coaly nucleus of appreciable thickness, even in the flattened specimens, and surrounded by a thin and veinless wing or margin. They have thus precisely the appearance of samaras of many existing forest-trees, some of which they also resemble in the outline of the margin, except that the wings of samaras are usually veiny. The character of the nucleus, and the occasional appearance in it of marks possibly representing cotyledons or embryos, forbids the supposition that they are spore-cases. They must have been fruits of phænogams. Whether they were winged fruits or seeds, or fruits with a pulpy envelope like those of cycads and some conifers, may be considered less certain. The not infrequent distortion of the margin is an argument in favour of the latter view, though this may also be supposed to have occurred in samaras partially decayed. On the other hand, their being always apparently flattened in one plane, and the nucleus being seldom, if ever, found denuded of its margin, are arguments in favour of their having been winged nutlets or seeds. Until recently I had regarded the latter view as more probable, and so stated the matter in the second edition of “Acadian Geology.” I have, however, lately arrived at the conclusion that the Cardiocarpa of the type of C. cornutum were gymnospermous seeds, having two cotyledons embedded in an albumen and covered with a strong membranous or woody tegmen surrounded by a fleshy outer coat, and that the notch at the apex represents the foramen or micropyle of the ovule. The structure was indeed very similar to that of the seeds of Taxus and of Salisburia. With respect to some of the other species, however, especially those with very broad margins, it still appears likely that they were winged.