The far-reaching Pennsylvanian swamplands had abundant species of trees and other plants that long since have become extinct. Today’s common deciduous trees were not present; flowering plants had not yet evolved. Instead, the tangled forests were dominated by giant ancestors of presently existing club-mosses, horsetails, ferns, conifers, and cycads. The undergrowth also was well developed, consisting mainly of ferns, fernlike plants, _Sphenophyllum_, and small club-mosses. The plant fossils give no indication of seasonal variations. The forests, evidently always green, grew rapidly and abundantly, with foliage of unprecedented size and luxuriance. Land animals were just beginning to develop and included sluggish, salamander-like amphibians, large primitive insects, and a few small reptiles. The insects flourished as never before or since in the damp forests and attained remarkable size. Insects more than four inches long were common and some are known to have been more than a foot long with a wingspread proportionately broad. Ancestors of the modern spiders, scorpions, centipedes (one fossil found in Illinois was twelve inches long), cockroaches, and dragonflies are represented by several hundred species. The fossilized plants of Pennsylvanian time belonged to only a few main categories: scale and seal trees, ancient scouring rushes (horsetails), herbaceous _Sphenophyllum_, ferns, seed ferns, and cordaitean trees. SCALE AND SEAL TREES (Plate 1) Scale and seal trees were abundant during the Pennsylvanian Period and were important contributors to coal beds. Although distantly related to the diminutive club-mosses and ground pines of the present, the trees grew on straight, slender trunks to heights of more than a hundred feet. [Illustration: Reconstruction of _Lepidodendron_ (after Hirmer)] Scale trees were so called because their numerous, closely set, spirally arranged leaves left scarred “cushions” on the branches and trunk, making them appear scaly. Seal trees derived their name from the signetlike appearance of their leaf cushions. The two best known types belong to the genera _Lepidodendron_ (scale tree) and _Sigillaria_ (seal tree), and fossils of both are common in Illinois. _Lepidodendron_ had long, slender, somewhat tapering trunks. Some of the trees reached heights of more than 100 feet and measured more than two feet in basal diameter. The trunk ended in a spreading crown formed by repeated dichotomous branching. The leaves were awl-shaped or linear, ranging from one to 30 inches long. The leaf cushions of _Lepidodendron_ are diamond-shaped, longer than broad, and arranged in spiral rows around the trunk and branches. A different name, _Lepidophyllum_, is used for fossils of the long, bladelike leaf when it is found detached.