5. As the species above described indicates, good external characters can be found for establishing species of this genus, and these species are of value as marks of geological age.
Genus Archæocalamites, Sternberg.
This genus has been established to include certain Calamites of the Devonian and Lower Carboniferous, in which the furrows on the stem do not alternate at the nodes or joints, and the leaves in one species at least bifurcate. C. radiatus, Brongniart, is the typical species. In North America it occurs in the Erian, probably as low as the Middle Erian. In Europe it has so far been recognised in the Lower Carboniferous only. I have, however, seen stems from alleged Devonian beds in Devonshire which may have belonged to this species.
Family Asterophylliteæ; Genus Asterophyllites, Brongniart.
Stems ribbed and jointed like the Calamites, but with inflated nodes and a stout internal woody cylinder, which has been described by Williamson. From the joints proceeded whorls of leaves or of branchlets, bearing leaves which differed from those of Calamites in their having a distinct middle rib or vein. The fructification consisted of long slender cones or spikes, having whorls of scales bearing the spore-cases. Some authors speak of Asterophyllites as only branches and leaves of Calamites; but though at first sight the resemblance is great, a close inspection shows that the leaves of Asterophyllites have a true midrib, which is wanting in Calamites.
Genus Annularia.—It is perhaps questionable whether these plants should be separated from Asterophyllites, The distinction is that they produce branches in pairs, and that their whorls of leaves are one-sided and usually broader than those of Asterophyllites, and united into a ring at their insertion on the stem. One little species, A. sphenophylloides, is very widely distributed.
Pinnularia—a provisional genus—-includes slender roots or stems branching in a pinnate manner, and somewhat irregularly. They are very abundant in the coal shales, and were probably not independent plants, but aquatic roots belonging to some of the plants last mentioned. The probability of this is farther increased by their resemblance in miniature to the roots of Calamites. They are always flattened, but seem originally to have been round, with a slender thread-like axis of scalariform vessels, enclosed in a soft, smooth, cellular bark.
Family Rhizocarpeæ; Genus Sphenophyllum.
Leaves in whorls, wedge-shaped, with forking veins. Fructification on spikes, with verticils of sporocarps. These plants are by some regarded as allied to the Calamiteæ and Asterophylliteæ, by others as a high grade of Rhizocarps of the type of Marsilia. The stem had a star-shaped central bundle of scalariform or reticulato-scalariform vessels.
Genus Sporangites. (Sporocarpon, Williamson.)