THE TEETH OF FISHES 87 



composed of vaso-dentin, — a soft dentin with a 

 circulation, — the point of the tooth only being cov- 

 ered with enamel in some forms. This dentin is 

 somewhat denser than the bone of the jaw, but 

 is not so dense as the dentin of the higher Verte- 

 brates. In some forms it is not covered, and in 

 others, as the Sharks, it is protected by a shiny 

 enamel-like substance, but this is not true enamel. 

 In other forms, as in the Sargus, the dentin is 

 yet harder, and is covered with a thick layer of 

 dense substance developed by a distinct organ. 

 Sometimes this enamel is covered by a layer of 

 cementum. In others again there is a mere os- 

 seous substance which results from the calcifica- 

 tion of the pulp. 



The Development and Succession of the Teeth 

 in Fishes. In the great majority of fishes the 

 germs of the teeth are developed directly from the 

 mucous membrane of the mouth, throughout the 

 v hole period of succession. This is peculiar to 

 the class. In all species the teeth are shed and 

 constantly renewed throughout the whole life of 

 the individual. Generally there is more than one 

 successional tooth developing, so that several 

 teeth are in process of formation destined to suc- 

 ceed one another in regular order. The mode of 

 succession varies with the method of attachment, 

 however, of which there are several in this great 

 class, — i.e., ankylosis, elastic hinge, and fibrous 



