THE TEETH OF FISHES 89 



be developed upon the roof of the mouth or palate, 

 or upon the tongue, or may be supported by pecul- 

 iarly developed labial cartilages. 



The Eiasmobranchii (the Sharks, Rays, etc.) 

 have the body covered — when covered at all — by 

 tooth-like structures which are small and close 

 set, and the layer is called Shagreen. When 

 larger and more scattered they form dermal 

 plates or tubercles, and when they take the form 



Fig. 30. — Teeth of Shark. 



of spines are called dermal defenses. These cov- 

 erings constitute the "placoid exo-skeleton," 

 which in minute structure precisely resembles 

 teeth. The protruding surfaces are frequently 

 ornamented with an elegant sculpturing of vari- 

 ous patterns. There are no true jaws, but in- 

 stead cartilaginous forms of the palato-quadrate 

 arch and of Meckel's cartilage. The teeth of 

 sharks are always very numerous and arranged 



