i66 



THE ORIGIN AND EVOLUTION OF LIFE 



obtain our first glimpses of North American land life in the 

 presence of the oldest known air-breathing animals, the scorpion 



spiders, also of the first known 

 land plants. There are indica- 

 tions of an arid climate in many 

 parts of the world. 



In Upper Silurian time the 

 ostracoderms attain the slow, 

 armored, bottom-living stage of 

 evolution, typified in the ptera- 

 spidians and cephalaspidians, 

 which were widely distributed 

 in Europe, in America, and pos- 

 sibly in the Antarctic regions, 

 as indicated by recent explora- 

 tions there. Belonging to an- 

 other and very distinct order, or 

 subclass (Antiarchi), are certain 

 armored Devonian forms {Botli- 

 riolepis, Pterichthys, etc.), which 

 possessed a pair of jointed lat- 

 eral appendages. Some of 

 these fishes, which are propelled 

 by a pair of appendages at- 

 tached to the anterior portion 

 of the body, present analogies to 

 the eurypterids (Merostomata, 

 or Arachnida) . 



In the fresh-water deposits 

 of Lower Devonian age have 

 been discovered the ancestors of 

 the heavily armored fishes 



Fig. 48. The Arthrodira. 



(Above.) Restoration of the gigantic 

 Middle Devonian Arthrodiran (jointed 

 neck) fish Dinichthys intermedins, eight 

 feet in length, of the Cleveland shales 

 (Ohio), showing the bony teeth and 

 bony armature of the head region. 

 (Below.) Lateral view of the same. 

 Model by Dr. Louis Hussakof and ISIr. 

 Horter, in the American Museum of 

 Natural History. 



