Page:Popular Science Monthly Volume 77.djvu/155

Rh skeleton has brought about many morphological modifications from those shown in the Batrachia and Reptilia. We find the skull loses the prefrontal and postfrontal bones, the mandible is simplified, the limb bones show a development of terminal epiphyses with ossification to the center of the vertebras and the bones of the pelvic arch are ossified. From the beginning of the Tertiary time a marvelous variety of morphological characters appears, and without the fossil types we should have but an inadequate conception of this great phylum.

The contributions of paleobotany to morphology are in some respects quite as striking as those of paleozoology.

The fossil Thallophytes have not furnished any very striking variations from their present morphological features, while the Bryophytes are scarcely represented as fossils except in very recent deposits.

The remaining phyla, the Pteridospermatophytes, the Pteridopliytes and the Spermatophytes have their oldest known beginnings as far back as the Devonian and their study has enormously widened the bounds of plant morphology.

The Pteridospermatophytes, which are confined to the Paleozoic, are in habit and vegetative morphology ferns—in methods of reproduction and in the morphology of their reproductive organs typical seed plants. They alter our whole conception of ferns and seed plants and in their significance are comparable to archetypal vertebrata, the acquisition of the seed habit in plants and the vertebral column in animals probably marking the culmination of the transfer of vital activity from aquatic to terrestrial conditions.

In the Pteridophytes the extinct Paleozoic class, the Sphenophyllales, is significant, since the morphology of the distinct lycopod and Equisetum lines seems to merge in this group. The lycopod type, itself represented in the existing flora by six or seven genera of herbaceous plants, monotonously uniform in their morphology, is found in the Paleozoic to constitute one of the chief units in the arborescent flora with numerous species of complex organization, whose stem, foliar and reproductive morphology was quite unknown to botanists (Lepidodendron, Sigillaria, etc.). The Equisetum type furnishes a like case. With few existing species of minor importance and uniform morphology we find in the Paleozoic a host of forms, many of them arborescent and of varied and complex structure {Calamites, Archceocalamites, etc.). Similar examples could be drawn from the fossil representatives of the true ferns.

In the Spermatophytes another wholly extinct class, the Cordaitales, embraces a curiously organized group of conifers extending back to the oldest horizons from which land plants are found, and continuing to the close of the Paleozoic as one of the most abundant as well as the highest type of pre-Mesozoic plant. In the older Mesozoic we find two