PALEONTOLOGY 305 



the changes within isolated groups of a species may be parallel, 

 and that the specific changes in different groups may progress 

 with very different degrees of velocity. 



The earliest known vertebrate remains are found in rocks of 

 the Ordovician age, approximately of the epoch known as 

 Trenton, at Canon City, in Colorado. These remains consist 

 of broken bits of bony shields of mailed fishes or fishlike forms 

 known as Ostracophores. With these are fragments of scales, 

 which seem to belong to more specialized forms. It is evident 

 that these remains, as well as the remains of sharks which 



FIG. 180. An ostracoderm, Pterichyodes milleri, from the lower Devonian of Scotland. 

 The jointed appendage on the head is not a limb. (After Traquair.) 



appear later in the Upper Silurian, by no means reveal the 

 actual first existence of vertebrates. 



The sharks which appear in the Upper Silurian, although 

 certainly primitive, even as compared with later sharks, are 

 very far from the simplest even of known vertebrates. There 

 seems to be good reason for the view that the vertebrate type of 

 animal, with the nervous cord along the back and the alimentary 

 canal marked by gill slits, was at first soft-bodied and worm- 

 like, in fact, derived from a wormlike ancestry, and that, prior 

 to the Ordovician and Silurian time, it was devoid of hard 

 parts. The early sharks have teeth, and rough skin, fins, and 

 sometimes fin spines, all susceptible of preservation in the 

 rocks, even though the skeleton was soft and cartilaginous. 

 The Ostracophores, some of which, at least, seem to be modified 

 sharks, had no internal hard parts, but were protected by an 

 external coat of mail, perhaps formed of coalescent prickles or 

 scales. 



From the sharks were doubtless descended the group of 

 Fringe-fins or Crossopterygians, which were more distinctly 

 fishlike. From these, on the one hand by continuous speciali- 



