THE SILURIAN PERIOD 119 



Insects are not known from the Silurian. 



Vertebrates. — The only known Silurian Vertebrates were of 

 very simple types, such as the Ostracoderms and primitive Fishes, 

 probably Sharks. All of the Ostracoderms were small, odd-shaped 

 creatures, but pretty closely related to the more prolific Devonian 

 forms to be described later. 



