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 JNIaryland 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? We do 



