adductor process 

 Meckel's cartilage 



^ bony septum 



pulp cavity with ra 

 dentinal tubules 



B 



Figure 8-55. Teeth of the ray, Myliobatis. A, posterior or inner view of articulated jaws showing 

 plote-lii^e teeth in position; B, vertical section through a tooth plate; C, cross section through the crown 

 of tooth plate. (A after Owen, 1866, and Arambourg and Bertin, 1958; B after Rose, 1898; C, 

 after 0rvig (1951) 



of the other fish described (Figure 8-56). A dental lamina 

 extends into the margin of the jaw and gives off a succes- 

 sion of tooth buds. Each bud forms a cup-shaped enamel 

 organ overlying a papilla in which the crown of the tooth 

 forms. The enamel that covers the crown is relatively thin 

 and formed by calcification of the thickened and fibrous 

 basement membrane of the enamel organ. It is invaded by 

 the peripheral canals of the dentinal tubules and may even 

 enclose cell spaces. 



As the tooth develops it becomes larger. The pallial den- 

 tine may have circumpulpar dentine laid down inside it, 

 and the pulp cavity may be filled with osteodentine. A small 

 base of osteodentine or acellular bone completes the tooth. 

 As the tooth grows, it moves outward toward the margin of 

 the jaw and its point becomes free of the overlying tissues. 

 The tooth is used for a period of time as it continues to move 

 outward. When it reaches the outer margin of the jaw, it is 

 shed. There is usually a continual process of tooth replace- 



ives of mouth margin 



dent 



dental lamella 



omel organ 



Meckel's cartilage 



enamel organ 



3 O O <7 ' 



Meckel's cartilage 



B 



Figure 8-56. Development of teeth in sharks, Spinax niger. A, and Squo/us acanthias, B. (A after 

 Peyer, 1937; B after Kendall, 1947) 



TEETH 



247 



