THE TEETH OF VERTEBRATES 79 



temporo-mandibular articulation does not assume 

 a definite shape until after the teeth are in occlu- 

 sion. 



The Succession of the Teeth in Vertebrates. 

 The Fishes and Reptiles have the teeth supplied 

 in endless succession. When teeth are lost, oth- 

 ers rise to take their places by various processes. 

 In the Sharks and Bays, the teeth are supported 

 by a fibrous membrane which carries them up 

 over the edge of the jaw (Fig. 28), bringing new 

 ones into place as the old teeth are lost. Hinged 

 teeth are replaced as lost by new ones which are 

 constantly developed at the base of the old teeth. 

 Ankylosed teeth are replaced by the successive 

 teeth being raised up beside or in the place of the 

 old teeth, and new bone of attachment is repro- 

 duced to support them. Socket teeth are replaced 

 by the advance of new teeth from within the body 

 of the jaw when the old teeth are lost, as in man. 

 In the Fishes and Eeptiles many sets of teeth are 

 developed during the life of the individual. In 

 the higher forms the teeth arise de novo from the 

 mucous membrane. The number is constant and 

 indefinite. 



In the present Mammalia there is generally but 

 two sets of teeth produced. The Elephant has 

 six molars, each one of which may be taken as a 

 set of teeth, as but one erupts at a time and never 

 more than two are present at one time. Mam- 



