590 GEOLOGY 



armor of some of them, and because of their intimate association 

 with true fishes, they were formerly classed as fishes. But no 

 vertebrae (or notochord) has been found, or appendages or jaws 

 of the vertebrate type. On the contrary, each jaw consists of two 

 separate parts which act upon one another, not vertically, in ver- 

 tebrate fashion, but laterally, as in arthropods. This is especially 

 true of the upper jaw. The ostracoderms probably formed the 

 climax and almost the end of their own strange race, for they practi- 

 cally disappeared with this period. Their disappearance is not 

 surprising in view of the development of powerful fishes, for the 

 ostracoderms were obviously not a masterful race. Besides being 

 small, they were clumsy, their locomotive organs lacked flexibility 

 and efficiency, and their mouth-parts were weak. They probably 



Fig. 422. Palceospondylus gunni, restored by Traquair; from the Old Red 

 Sandstone, Caithness, Scotland. (After Dean.) 



plowed the soft bottoms of the sluggish waters, half buried in the 

 mud, above which little beside their peculiarly placed eyes and the 

 backs of the plated bucklers were habitually exposed. While the 

 ostracoderms are sometimes reported as occurring in beds with 

 marine fossils, there is little evidence that they were dwellers in 

 the open sea. 



In view of the arthropodan relations of the ostracoderns, it is 

 suggestive that in the Devonian time, as in the closing stages of 

 the Salina, there were gigantic eurypterids associated intimately 

 with the ostracoderms and the fishes. The largest of the art lire >p< >< l> 

 (Pterygotus) reached the extraordinary length of two meters. It 

 has already been suggested that this association of arthopods and 

 vertebrates ran back to their origin, and it may be added thai 

 it has run on until the present day, for the fish and the arthropods 

 (crayfish and smaller crustaceans) dominate the fresh waters. 



