THE IMPENETRABLE SEA 



precision, and directed backwards. Sharkskin, com- 

 mercially known as shagreen (although the term covers 

 many kinds of grained leather prepared from the skins 

 of animals, including wild asses, camels, sea-otters, and 

 seals, besides sharks) is used in the manufacture of hand- 

 bags, purses, spectacle-cases, etc. 



Our own teeth are embedded in the bone of the jaw, 

 but shark's true teeth (those in their mouths) are set in 

 their gums. They may be sharply pointed and separate, 

 or blunt and articulated together, so as to form pave- 

 ment-like structures. 



In some species the teeth roll over each other as the 

 shark's mouth closes, like the cylinders in a crushing mill, 

 producing a grinding effect of enormous power. The 

 sharks known as the smooth hound, ray-toothed dog and 

 skate-toothed varieties have these peculiar grinders, 

 which are rendered necessary by the food on which they 

 live: such as hard-shelled molluscs and crustaceans, 

 whose armour is ground under the bony rollers. 



Sharks' teeth resemble those of the whelk in the fact 

 that they are replaceable, but while the whelk has its 

 teeth on a ribbon the shark's are individually renewable. 

 Each tooth has other teeth beneath it, so that a con- 

 tinuous succession is provided for every tooth as it wears 

 away or breaks off. A single tooth may be renewed more 

 than a hundred times in a shark's lifetime. One might 

 think that because the teeth are set in the creature's gums 

 they would not be so strong as if fixed in their jaws; but 

 there cannot be much wrong with the arrangement con- 

 sidering that sharks can bite through steel hawsers and 

 tear the flesh of dead whales to ribbons as though they 

 had been ripped by circular saws. 



The white shark's teeth form one of the most ex- 

 traordinary structures possessed by any animal for tear- 

 ing and grinding its food. Crocodiles and other creatures 

 can renew their teeth, but the white shark is far more 

 efficient. If the shark is an adult it has in its upper and 



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