CHAPTER II 



THE DENTITION OF LOWER VERTEBRATES 



The relation of teeth to scales — Attachment of teeth — Teeth 

 of Fishes, Amphibians and Reptiles — Evolution of "cusps" 

 — The pronounced development of certain teeth. 



Teeth as popularly understood may be defined as calcined 

 papilla? of the skin secondarily implanted in the bones of the 

 jaws and subserving the purpose of mastication. That this is 

 but a crude conception will appear upon the slightest reflection 

 for it is based only upon certain generally recognized features 

 of our own dentition and one cannot think for a moment that 

 our teeth sprang into existence fully formed and adapted at 

 once to their present function. It is indeed only after a long 

 period of evolution, of constant modification and adaptation to 

 circumstances that the present form and characters of our teeth 

 have developed. To understand even our own dentition intel- 

 ligently it is necessary to trace it back to its earliest beginning 

 and to follow carefully the successive steps by which it attained 

 its present condition. Upon discussing our definition more in 

 detail Ave shall find that it becomes merely a generalization 

 more or less exact and of limited applicability. 



If we examine the jaws of a Dogfish or Shark (Fig. 3) we 

 shall observe that the horny scales of the skin are continued 

 over the framework of the jaws into the mouth and that on the 

 rounded surface of the jaws these scale-like structures assume 

 the form of simple triangular teeth in successive rows. Hence 

 we shall be less surprised at the suggestion that the teeth are 

 in reality modified dermal appendages. But if next Ave take the 

 head of a Pike (Fig. 4) Ave do not see this simple transition and 

 further Ave note the appearance of small but distinct tooth strue- 



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