THE TEETH OF FISHES. 221 



by a rotation of the mucous membrane over the surface 

 of the jaw, so that, as might have been expected, large 

 numbers of the isolated fossil teeth of Cestracionts are to be 

 met with. 



The teeth of the Cestracion are fitted for the trituration 

 of hard substances, and for such they are used, its food 



FIG. 94 !. 



consisting of shell-fish, &c. The teeth consist of vaso- and 

 osteodentine, protected by what is apparently a structure- 

 less layer of enamel. 



The extinct Cestracionts extended far back in time, being 

 met with in palaeozoic strata, and they were equally widely 

 distributed in space ; the size of many of the teeth also 

 indicates the existence of forms much larger than the recent 

 timid and inoffensive Cestracion Philippi. Many of the 

 extinct forms are known only by isolated teeth ; of others 



(*) Lower jaw of Cestracion Philippi. a. Young teeth not yet in use. 

 b. Large grinding back teeth, c. Small pointed front teeth. 



The new teeth are developed at the bottom of the series on the inner 

 side, and, just as in other sharks, are covered in by a flap of mucous 

 membrane. 



