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. 



