102 HISTORICAL GEOLOGY 



Vertebrates. — From the standpoint of evolution, perhaps the 

 most significant feature of the Ordovician is the occurrence of the 

 earliest known vertebrates. These were very primitive fishlike 

 forms such as Ostracoderms, which have been found in Ordovician 

 strata at certain places in Colorado and Wyoming. The fossils 

 are mostly very fragmentary, consisting chiefly of scales or plates, 

 but some nearly complete dermal plates are known. They strongly 

 suggest the Ostracoderms, but since such forms are much better 

 known from the Devonian, we shall postpone a fuller discussion 

 of these curious creatures. 





