But let us consider the actual forms of vegetation presented to us in the Coal period, as we can restore them from the fragments preserved to us in the beds of sandstone and shale, and as we would have seen them in our imaginary excursion through the Carboniferous forests. To do this we must first glance slightly at the great subdivisions of modern plants, which we may arrange in such a way as to give an easy means for comparison of the aspects of the vegetable kingdom in ancient and modern times. In doing this I shall avail myself of an extract from a previous publication of my own on this subject. "The modern flora of the earth admits of a grand twofold division into the _Phænogamous_, or flowering and seed-bearing plants, and the _Cryptogamous_, or flowerless and spore-bearing plants. In the former series, we have, first, those higher plants which start in life with two seed-leaves, and have stems with distinct bark, wood, and pith--the _Exogens_; secondly, those similar plants which begin life with one seed-leaf only, and have no distinction of bark, wood, and pith, in the stem--the _Endogens_; and, thirdly, a peculiar group starting with two or several seed-leaves, and having a stem with bark, wood, and pith, but with very imperfect flowers, and wood of much simpler structure then either of the others--the _Gymnosperms_. To the first of these groups or classes belong most of the ordinary trees of temperate climates. To the second belong the palms and allied trees found in tropical climates. To the third belong the pines and cycads. In the second or Cryptogamous series we have also three classes,--(1.) The _Acrogens_, or ferns and club-mosses, with stems having true vessels marked on the sides with cross-bars--the Scalariform vessels. (2.) The _Anophytes_, or mosses and their allies, with stems and leaves, but no vessels. (3.) The _Thallophytes_, or lichens, fungi, sea-weeds, etc., without true stems and leaves.