THE EARLIEST KNOWN VERTEBRATES 31 
examples we learn just what teeth and spines 
go with one another, and sometimes find that 
one fish has received names enough for an en- 
tire school. 
These ancient sharks were not the large and 
powerful fishes that we have to-day — these 
came upon the scene later — but mostly fishes 
of small size, and, as indicated by their spines, 
fitted quite as much for defence as offence. 
Their rise was rapid, and in their turn they 
became the masters of the world, spreading 
in great numbers through the waters that cov- 
ered the face of the earth; but their supremacy 
was of short duration, for they declined in 
numbers.even during the Carboniferous Period, 
and later dwindled almost to extinction. And 
while sharks again increased, they never reached 
their former abundance, and the species that 
arose were swift, predatory forms, better fitted 
for the struggle for existence. 
