254 



AN INTRODUCTION TO ZOOLOGY. 



formerly called placoid scales, and they are comparable 

 with the primitive form of ieei/t ; for there are small 

 teeth in some of the Selachii that exist as similar 

 ossifications in the mucous membrane of the mouth, 

 and the latter is only a continuation of the outer skin. 

 That all teeth have originated from this simple form 

 is proved from the way in which teeth arise in the 

 embryo animal, namely, as papillae (fig. 92), which after- 

 wards become hardened by deposits of hard material. 

 Larger teeth are also found in many Selachians, which 

 are believed to have been derived from the fusion 



I'ig; 91.— Jaws of Skate, showing multiple rows of teeth. 



of the smaller kind, and to be comparable with the 

 teeth of higher types (fig. 91). In sharks there are a 

 number of these teeth arranged in rows one inside 

 the other, so that the front row is most used in biting. 

 Such a fish has a practically unlimited number of 

 sets of teeth, the back rows coming into use when 

 the front ones wear down. In the two successive 

 sets of teeth possessed by mammals (milk teeth and 

 second teeth) it is believed that we see a reduced 

 representation of these successive rows, the number 

 being reduced to two, and these so modified by their 



