Page:The Scientific Monthly vol. 3.djvu/608

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ORIGIN AND EVOLUTION OF LIFE

��In each of these four or five great radiations of form, from the swift- moving to the bottom- or ground-living, slow, armored types, there is

usually an increase of bodily

size, also an increase of spe- cialization, the maximum in both being reached just before the period of extinction arrives. The Ordovician Ostraco- derms are very little known. The Upper Silurian Ostraco- derma enjoyed a wide distribu- tion in Europe and America. They include both the fusiform free-swimming type {Birkenia) and the depressed ray-like types {Lanarkia, etc.). They appar- ently had not yet acquired car- tilaginous lower jaws and ap- pear to be in a lower stage of and ■ evolution than the true Sshes. The armature is arranged in shield and plate form in Palw- aspis, from the Upper Silurian Salina time of Schuchert, where we obtain our first glimpse of North American land life in the presence of the oldest known air-breathing animals, the scorpion spiders, and also the first known land plants- There are many indications of an arid climate. In Upper Silurian

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��time the Ostracoderms reach the slow, armored, bottom-living stage of evolution, as typified iu the Fteraspidians and Cephalaspidiane, which were widely distributed in Europe, in America, and possibly in the Antarctic regions, as indicated by recent e-xplorations there. Be- longing to a very distinct order or subclass (Antiarehi) are certain armored Devonian forms {Botkriolepis, Pterichthys, etc.), which poB- aessed a pair of jointed lateral appendages. Propelled by a pair of appendages attached to the anterior portion of the body ; some of those animals {Botliriolepis) present analogies to the eurypterids (Meros- tomata, or Arachnida).

In the freshwaters of the Lower Devonian have been discovered the

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