RULERS OF THE ANCIENT SEAS 67 
supply of teeth never ran short. Back of each 
tooth, one behind another arranged in serried 
ranks, lay a reserve of six or seven smaller, but 
growing teeth, and whenever a tooth of the 
front row was lost, the tooth immediately 
behind it took its place, and like a well-trained 
soldier kept the front line unbroken. Thus 
the teeth of sharks are continually developing 
at the back, and all the teeth are steadily 
pushing forward, a very simple mechanical 
arrangement causing the teeth to lie flat until 
they reach the front of the jaw and come 
into use. 
Once fairly started in life, these huge sharks 
spread themselves throughout the warm seas 
of the world, for there was none might stand 
before them and say nay. ‘They swarmed 
along our southern coast, from Maryland to 
Texas; they swarmed everywhere that the water 
was sufficiently warm, for their teeth occur in 
Tertiary strata in many parts of the world, and 
the deep-sea dredges of the Challenger and 
Albatross have brought up their teeth by scores. 
And then — they perished, perished as utterly . 
as did the hosts of Sennacherib. Why? Wedo 
oy 
