THE EARLIEST KNOWN VERTEBRATES 25 



wise related, and the similarity is in appearance 

 only. 



Pterichthys, the wing fish, was another small, 

 quaint, armor-clad creature, whose fossilized re- 

 mains were taken for those of a crab, and once 

 described as belonging to a beetle. Certainly 

 the buckler of this fish, which is the part most 

 often preserved, with its jointed, bony arms, 

 looks to the untrained eye far more like some 

 strange crustacean than a fish, and even natu- 

 ralists have pictured the animal as crawling 

 over the bare sands by means of those same 

 arms. These fishes and their allies were once 

 the dominant type of life, and must have 

 abounded in favored localities, for in places are 

 great deposits of their protective shields jum- 

 bled together in a confused mass, and, save 

 that they have hardened into stone, lying just 

 as they were washed up on the ancient beach 

 ages ago. How abundant they were may be 

 gathered from the fact that it is believed their 

 bodies helped consolidate portions of the strata 

 of the English Old Red Sandstone. Says INIr. 

 Hutchinson, speaking of the Caithness Flag- 

 stones, " They owe their peculiar tenacity and 



