108 



THE VERTEBRATE SKELETON. 



(3) By implantation in sockets. This method is not very 

 common among fish. The teeth are sometimes, as in -Lepi- 

 dosteus, ankylosed to the base of the socket. In this genus 

 there is along each ramus of the mandible a median row of 

 large teeth placed in perfect sockets, and two irregular lateral 

 rows of small teeth ankylosed to the jaw. 



Dentine, enamel and cement are all represented in the teeth 

 of fishes, but the enamel is generally very thin, and cement is 

 but rarely developed. Dentine forms the main bulk of the 

 teeth; it is sometimes of the normal type, but generally differs 

 from that in higher vertebrates in being vascular, and is 

 known as vasodentine. A third type occurs, known as osteo- 

 dentine ; it is traversed by canals occupied by marrow, and is 

 closely allied to bone. 



FIG. 15. PAKT OF THE LOWEB JAW OF A SHABK (Galeus) 

 (from OWEN after ANDREA. 



1. teeth in use. 



2. reserve teeth folded back. 



3. part of the caudal spine of 



a Sting-ray (Trygon) which has 

 pierced the jaw and affected the 

 growth of the teeth. 



The teeth are generally continually renewed throughout life, 

 but sometimes one set persists. 



The teeth of Selachii are fundamentally identical with 



