88 COMPARATIVE DENTAL. ANATOMY 



membrane. In ankylosed teeth, the most common 

 form is for the new tooth to rise up nnder the 

 old tooth and by absorption displace it; others 

 displace the old tooth from the side by absorbing 

 its supporting cone. Hinged teeth are developed 

 in the folds of the mucous membrane, and as the 

 old teeth are lost through use, new ones rise up 

 to take their places. In the fishes having fibrous 

 membrane attachment, the teeth are developed in 

 the thecal fold on the inner side of the base of 

 the jaw, and are perfected in growth as they are 

 carried upward over the edge of the jaws by the 

 membrane in which they are imbedded. Thus in 

 the sharks the whole phalanx of the numerous 

 teeth is ever marching slowly forward in rotary 

 progress over the occlusal borders of the jaws, 

 the teeth being successively cast off after having 

 reached the outer margin and fulfilled for a longer 

 or shorter period their appointed function, and 

 are succeeded by others that rise up continually 

 to take the places of those that are lost. 



Descriptive. In the lowest forms of the car- 

 tilaginous fishes, as the Myxines, Lampreys, and 

 other parasitic types, we find some destitute of 

 true calcified teeth, but instead possessed of a 

 horny structure of a conical sharp-pointed and 

 often slightly recurved form. Some, as the Hag- 

 fish, have a lance-like tooth for piercing the vic- 

 tim to suck the blood. In others, horny teeth may 



