represented only by small, mostly delicate, trailing plants familiarly known as "club mosses" and "ground pines." The most conspicuous trees of the great Pennsylvanian lowlands and swamps have, indeed, left meager modern representatives, and here we have an excellent illustration of a once prominent group of plants which has dwindled away almost to extinction. Another common type of Pennsylvanian vegetation was the so-called "horsetail" plant or giant rush. The much smaller scouring rush, represented by several species to-day, is the direct descendant of this type which, during later Paleozoic time, grew to be 50 to 90 feet high and 1 to 2 feet in diameter. The long, slender trunks filled with pith were segmented with variously shaped leaves arranged in whorls around the joints. A fine, vertical-fluted structure without leaf scars characterized the surfaces of the trunk. Recent study has shown that many of the Pennsylvanian plants, long classed as true ferns, were really "seed ferns," as described below. Many of the true ferns grew to be real trees up to fifty or sixty feet high, but all Paleozoic types were primitive in structure as compared with modern ferns. Very remarkable among the later Paleozoic plants were the Pteridosperms, represented by the so-called "seed ferns." These now wholly extinct plants seem to have formed the connecting link between the seedless, flowerless plants (Cryptogams) and the seed-bearing, flowering plants (Phanerogams), because they bore seeds but not flowers. Many of them were small and herbaceous, but others were tall trees, in general appearance resembling the tree ferns. "Seed ferns," which play such an important part in the evolution of plants, are not known to have existed after Paleozoic time. During the latter half of the Paleozoic era some very primitive types of flowering plants (Gymnosperms) existed. Most abundant of these were the so-called Cordaites, which were the tallest trees of the time, some having reached heights of over 100 feet. The upper portions only bore numerous branches supplied with many simple, parallel-veined, strap-shaped leaves up to six feet long and six inches wide. Excepting the pithy cores the trunks of these trees were of real wood covered with thick bark. Trees of this kind became extinct in the early Mesozoic era.