THE EARLIEST KNOWN VERTEBRATES 15 



in troublous times when there was literally a fight for 

 existence and only such as were well armed or well 

 protected could hope to survive. A parallel case exists 

 to-day in some of the rivers of South America, where the 

 little cat-fishes would possibly be eaten out of existence 

 but for the fact that they are covered some of them 

 very completely with plate-armor that enables them 

 to defy their enemies, or renders them such poor eating 

 as not to be worth the taking. The arrangement of the 

 plates or scales in the living Loricaria is very sugges- 

 tive of the series of bony rings covering the body of 

 the ancient Cephalaspis, only the latter, so far as we 

 know, had no side-fins ; but the creatures are in no wise 

 related, and the similarity is in appearance only. 



Pterichthys, the wing fish, was another small, quaint, 

 armor-clad creature, whose fossilized remains were 

 taken for those of a crab, and once described as belong- 

 ing 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 naturalists 

 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 jumbled 

 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 Mr. Hutch- 

 inson, speaking of the Caithness Flagstones, "They 

 owe their peculiar tenacity and durability to the dead 

 fishes that rotted in their midst while yet they were only 



