ribs, and generally at a distance from each other. The form of these scars is extremely variable, and, as in lepidodendron, their size enlarges in proportion to the growth of the trees. The identification of the species is therefore very difficult, especially when it has to be made, as is generally the case, from fragmentary specimens. It is rendered still more embarrassing by the fact that the bark of some species of sigillaria is composed of different thin layers, easily separated, on which the leaf scars, though superposed, differ upon each layer. The club mosses of our time rarely have stems half an inch in diameter; those of the coal, or the lepidodendron and sigillaria, were represented by trees sometimes 2 ft. in diameter and 40 to 60 ft. high.
Fig. 8.—1. Stigmaria with leaves. 2. Stem of Stigmaria with scars. 3. Leaf of Stigmaria.
The stigmaria are comparable to the sigillaria by their texture; but they were merely floating stems, which, suspended in water by long hollow leaves, had a mode of life independent from that of their flower- or fruit-bearing stems, the sigillaria, lepidodendron, &c., which came out only when a kind of ground had been formed and consolidated by the growth of the stigmaria. The connection of these floating stems with trunks of sigillaria and of lepidodendron has disposed many palæontologists to consider them as mere roots. But besides many other considerations, the fact that these stigmaria did live independently and for long periods without any sigillaria stems, even forming by their debris thick strata of clay, is a sufficient reason for recognizing them as true stems. In the peat bogs of our time there is a small club moss, lycopodium inundatum, which in a di- minutive way shows us the mode of life of those stigmaria. It expands all around upon the soft muddy surface of the bogs, its creeping stems interlaced in every direction; and when the carpet has become solid enough, the flowering stems come up and ripen their seeds out of reach of the water. The stigmaria trunks do not vary much in size from 2 to 4 in. in diameter, but they indefinitely extended their branches by forking, and with their long leaves formed a floating network, soon solid enough to support large trees. The same result is now produced in our peat bogs by the vegetation of the floating mosses. All the scars left at the point of attachment of the leaves of stigmaria upon their stems are almost exactly round, like a swollen convex surface one and a half to two lines in diameter, surrounded by a ring and pierced in the centre by a point, the scar of a vessel. The relative position of the scars is more generally in quincuncial order, at a distance of an inch or less; they are sometimes connected by star-like wrinkles, or separated in undulate rows by narrow ridges.
Fig. 9.—1. Lepidostrobus. 2. Cross section of a small species. 3. Sporange and blade of Lepidostrobus.
The fruits of the lycopodiaceous species of the carboniferous, lepidostrobus, are in the form of seeds enclosed in receptacles or spore cases in right angle to a common axis, and forming cones covered with imbricated scales. Their form and arrangement are like those of the club mosses, but the size of the cones is proportionate to that of the trees. They vary from 1 to 2 in. in diameter, and from 2 to 18 in. or more in length.
Fig. 10.—Calamites.
—The calamites, which have left very abundant remains in the coal, are closely related by their hollow articulated stems with bark vertically and equally fluted, by their fructifications, and by their structure, to living species of equisetum (horse-
