THE TEETH OF FISHES. 219 



the teeth of these two rows, both above and below it, being 

 stunted and smaller than their neighbours. 



Hence the development of these teeth, which ultimately 

 came to be at some little distance from the spine, had been 

 profoundly modified by its presence, and it is difficult to 

 understand in what manner this could have affected them 

 had they not, at an earlier period of their growth, lain in, 

 more immediate proximity to it. But if the membrane, 

 with the teeth attached, does move slowly along the surface 

 of the jaw, this difficulty at once disappears. 



The forms of the teeth in various sharks are different 

 and characteristic ; nevertheless they vary somewhat with 

 age in some species, and present differences in size and 

 form in the upper and lower jaws, or in different parts of the 

 mouth of the same individual. For instance, in Lamna, in 

 the upper jaw, the third teeth of each horizontal row, count- 

 ing from the middle line, are very small, while in both jaws- 

 there is a gradual diminution in the size of the teeth towards 

 the back of the mouth. 



Thus, although it is often possible to refer a particular 

 tooth to its right genus or even species, much care is re- 

 quisite in so doing. 



The teeth of the bloodthirsty white shark (Carcharias) 

 are triangular flattened plates, rounded on their posterior 

 aspect, with trenchant slightly serrated edges ; it is pointed 

 out by Professor Owen that if the relation between the size 

 of the teeth and that of the body were the same in extinct 

 as in recent sharks, the dimensions of the teeth of the 

 tertiary Carcharodon would indicate the existence of sharks. 

 as large as whales. 



The intimate relationship between the teeth and the 

 dermal spines, which from the standpoint of development, 

 has been illustrated at page 2 and page 119, is apparent also 

 in their histological structure. There are many dermal 

 spines to be met with in the sharks, which seen alone could 



