PLAGIOSTOMI. 



107 



largest teeth. As the teeth increase in size, they diminish in 

 number in each row. The series of the largest teeth includes 

 from six to seven in the upper, and from seven to eight in the 

 lower jaw. Behind this row the teeth, although preserving 



Fig. 27 a. 

 Upper jaw and teeth of Port-Jackson Shark (Cestracion), half nat. size. 



their form as crushing instruments, progressively diminish in 

 size, while at the same time the number composing each row 

 decreases. From the oblique and apparently spiral disposi- 

 tion of the rows of teeth, their symmetrical arrangement on 

 the opposite sides of the jaw, and their graduated diversity of 

 form, they constitute the most elegant tesselated covering to 

 the jaws which is to be met with in the whole class of fishes. 

 The modifications of the form of the teeth above described, 

 by which the anterior ones are adapted for seizing and re- 

 taining, and the posterior for cracking and crushing alimentary 

 substances, are frequently repeated, with various modifications 



