TEETH AND FOOD 



121 



and the lips are formed that this continuity is broken, the 

 denticles in the region of the jaws becoming enlarged and 

 taking on the distinctive characters of teeth, while those on the 

 outside of the body remain more or less unchanged. As in the 

 case of the denticles already described, each tooth consists 

 mainly of a bony substance known as dentine, with an internal 

 pulp cavity and an outer covering of enamel. Thus, the teeth 



B 



Fig. 5°- 



A. Cross-section through the lower jaw of an embryo Spotted Dog - fish 

 (Scyliorhinus sp.), showing the gradual transition from dermal denticles (d) on 

 the outer surface to teeth (t) on the inner surface. The dotted area in the 

 centre represents the cartilage of the lower jaw. Greatly enlarged. (After 

 Gegenbaur) ; b. Cross-section through the lower jaw of Sand Shark 

 (Odontaspis taurus), showing succession of teeth, X ^. 



of all fishes, and indeed of all higher vertebrates, including 

 man himself, owe their origin to the modification of the dermal 

 armour of the skin, ' and the teeth are to be found in their 

 simplest form in the Selachians. 



There is no firm attachment of the teeth to the jaws in these 

 fishes, and they are simply embedded in the gums. Nor does a 

 Shark or Ray retain the same teeth throughout the greater 

 part of its life, but as those in use become worn and useless 

 they are replaced by new ones. An examination of the mouth 



