THE INCREDIBLE SHARKS 339 



It is these skin denticles which give us the clue as to the 

 method by which chewing, grinding and cutting teeth were 

 evolved. The gradual transition from skin denticles to actual 

 teeth is readily observed in the embryos of certain sharks, 

 where, if one follows with a glass the curve of the mouth from 

 exterior to interior, the denticles become more and more al- 

 tered, larger, and finally perfect and functional teeth. The 

 peculiar teeth on the swords of sawfish, which are close rel- 

 atives of the sharks, are derived similarly from the skin. These 

 teeth are greatly enlarged skin denticles which have grown in 

 alternate positions on the blade of the sword. The teeth in the 

 mouth of a shark are frequently laid in series, row after row, 

 sometimes to the amount of four hundred. These advance con- 

 tinually, replacing those worn out or lost. Only the outer 

 series are used. 



Depending on their usage, the teeth of shark and their rela- 

 tives show a great variety of form. Some of these are little more 

 than tiny pointed needles, others have become blunt, or even 

 hexagonal while some are triangular with razor-sharp saw edges 

 that cut like a knife. A bite from one of the latter would shear 

 flesh into ribbons. Strangely, one of our very largest sharks, the 

 whale sharks, which reach a length of forty feet and a recorded 

 weight of 26,500 pounds, have teeth which are on the average 

 less than a quarter of an inch long. These fish are quite without 

 means of protection and certainly, in spite of their huge bulk, 

 are among the most inoffensive of creatures. I once saw one 

 of these huge fellows basking in the sunlight on top of the 

 ocean about half way between Inagua and Haiti. It was beauti- 

 fully checkered with a series of light polka dots and rectangu- 

 lar cross lines. The ship missed it; by inches. A moment later 

 the monster sounded and went hurtling into the depths. Little 

 is known of this species and it has been seen only a few times. 

 Its food is believed to consist of small plankton, organisms that 



