Göppert, established in his Genera, where he especially considers fructifications, and that of Ettinghausen, which fixes with great detail the character of the groups and their subdivisions from similarity of nervation. These different systems are interesting to palæontologists, but present all the same insufficiency resulting from the difficulty of observing the fructification, or from the similarity of the nervation in different species or even genera. Five or six genera of living ferns, for example, exactly limited by the characters of their fructifications, all present in some of their species the same kind of nervation. It is worth remarking, however, that the fossil ferns of the carboniferous measures are in this country more generally found in fruiting specimens than in Europe.
Fig. 5.—Caulopteris Worthenii.
The same remark may be applied to the trunks of fern trees, whose scarcity has been generally remarked in Europe as a fact seemingly in contradiction with what should be expected in considering the great size of the other kinds of vegetables of the carboniferous period. In the coal measures of the United States, these trunks of ferns, named caulopteris, stemmatopteris, psaronius, &c., and generally classed by the characters of their internal structure or by the scars of leaves upon the bark, are not rare, indeed are extremely abundant in some localities. In southern Ohio and northern Virginia, trunks of ferns are imbedded in a thick bed of sandstone occupying a wide area. These trunks, mostly silicified, are either standing or prostrated, and their fragments are now strewn along the creeks, which by their course through this region have taken off and displaced these remains. They are not only in great number, but some of them are large. The average diameter is 8 in., but some measure more than a foot. A splendid specimen in the museum of comparative zoölogy at Cambridge is 2 ft. high and 14 in. in diameter, and almost exactly cylindrical. Generally, however, in connection with the roof shale of coal beds, the ferns are mostly bushy or herbaceous species, the shale being composed of soft muddy materials, forming a ground too unstable for the vegetation of large trees.
Fig. 6.—1. Lepidodendron. 2. Scars of a larger branch.
—The club moss or lycopod family is represented in the carboniferous formation mostly by large trees and floating stems, also proportionately large. They belong to several genera, of which the more remarkable are the lepidodendron, sigillaria, and stigmaria. These genera are mostly characterized by the impressions of the leaves forming at the point of attachment rhomboidal, oval, or round scars, which, small in young branches and young stems, enlarge after the fall of the leaves in proportion to the growth of the trees. In species of lepidodendron these scars are placed spirally along the trunk and contiguous to each other. In the small branches or immediately after the parting of the leaves these scars are scarcely half a line in diameter on each side; in old trees of the same species the scars are an inch and a half long and an inch broad. They form either surface or flat impressions, or are more or less deeply cut into the bark.
Fig. 7.—Sigillaria.
The sigillaria species differ essentially from the lepidodendron by the position of the scars, which are placed in vertical rows separated by convex or flat
