There were also vast monotonous forests, composed chiefly of trees belonging to the lycopods, and whose nearest relatives to-day are the little club mosses of our eastern woods.  Two families of lycopods deserve special mention,—­the Lepidodendrons and the Sigillaria.

The Lepidodendron, or “scale tree,” was a gigantic club moss fifty and seventy-five feet high, spreading toward the top into stout branches, at whose ends were borne cone-shaped spore cases.  The younger parts of the tree were clothed with stiff needle-shaped leaves, but elsewhere the trunk and branches were marked with scalelike scars, left by the fallen leaves, and arranged in spiral rows.

The Sigillaria, or “seal tree,” was similar to the Lepidodendron, but its fluted trunk divided into even fewer branches, and was dotted with vertical rows of leaf scars, like the impressions of a seal.

Both Lepidodendron and Sigillaria were anchored by means of great cablelike underground stems, which ran to long distances through the marshy ground.  The trunks of both trees had a thick woody rind, inclosing loose cellular tissue and a pith.  Their hollow stumps, filled with sand and mud, are common in the Coal Measures, and in them one sometimes finds leaves and stems, land shells, and the bones of little reptiles of the time which made their home there.

It is important to note that some of these gigantic lycopods, which are classed with the CRYPTOGAMS, or flowerless plants, had pith and medullary rays dividing their cylinders into woody wedges.  These characters connect them with the PHANEROGAMS, or flowering plants.  Like so many of the organisms of the remote past, they were connecting types from which groups now widely separated have diverged.

Gymnosperms, akin to the cycads, were also present in the Carboniferous forests.  Such were the different species of CORDAITES, trees pyramidal in shape, with strap-shaped leaves and nutlike fruit.  Other gymnosperms were related to the yews, and it was by these that many of the fossil nuts found in the Coal Measures were borne.  It is thought by some that the gymnosperms had their station on the drier plains and higher lands.

The Carboniferous jungles extended over parts of Europe and of Asia, as well as eastern North America, and reached from the equator to within nine degrees of the north pole.  Even in these widely separated regions the genera and species of coal plants are close akin and often identical.

Invertebrates.  Among the echinoderms, crinoids are now exceedingly abundant, sea urchins are more plentiful, and sea cucumbers are found now for the first time.  Trilobites are rapidly declining, and pass away forever with the close of the period.  Eurypterids are common; stinging scorpions are abundant; and here occur the first-known spiders.