TEETH 



Teeth 



27 



Teeth have long been known to represent the dermal 

 defences of the mouth rim. In this region they have 

 become of especial value in the living economy of verte- 

 brates seizing, holding, cutting, or crushing the food- 

 material. They have here accordingly been retained and 

 specialized. In the sharks the dermal denticles of the 

 mouth rim are often identical in shape and pattern with 

 those of the entire body surface : they differ only in 

 their larger size. Their arrangement in many rows still 

 presents clearly their metameral character. 



The forms of teeth acquired among the different groups 

 of fishes suggest closely the evolution of the more modi- 

 fied dermal defences. In general, they are found to vary 

 widely according to their function or location ; those near- 

 est the dermal margin of the mouth usually retaining 

 the cusp-like and more primitive features. Thus in the 

 jaw of Port Jackson shark (Fig. 27, v. p. 85), the teeth of 

 the symphysial region clearly represent shagreen denti- 

 cles ; while those deeper in the mouth, large and blunt, 

 serve as crushing or "pavement " teeth. These must evi- 

 dently be looked upon as standing in the same relation to 

 the anterior cusps, as do the bone plates of Fig. 25 to the 

 derm denticles of Fig. 23 ; the fused crushing teeth have 

 still retained their metameral arrangement. The dental 

 plates (Fig. 30) of a ray, Myliobatis (p. 96) show more 

 perfect conditions for crushing ; they are uniform in size, 

 tightly set, and present a smooth, mosaic-like surface. A 

 still more perfect fusion of the dental elements occurs in 

 a ray, closely akin to Myliobatis ; all lateral elements have 

 here been fused, but their metameral sequence has been re- 

 tained (Fig. 29). In Fig. 28 is shown a dental plate of a 



