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 Mr. 
Hutchinson, speaking of the Caithness Flag- 
stones, “They owe their peculiar tenacity and 
