TEETH 27 
Teeth 
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 
A 
