Figure 297 .—Diodon holocanthus: cross sectioi 



of upper (above in each set) and lower jaws 



just to one side of the midline, showing 



the numbers of dental units in the biting 



edges of the jaws and in the trituration 



plates, increasing greatly in number with 



increasing specimen size: A, 63.4 mm SL; 



B, 122 mm SL; C, 183 mm SL; D. 375 mm SL. 



but there is little difference between species in the num- 

 ber of plates at any given size, based on the limited sur- 

 vey made (Fig. 302). 



However, the largest specimen of a diodontid exam- 

 ined for internal trituration teeth, a single 545 mm spec- 

 imen of C. reticulatus, had only about 18 plates, ap- 

 proximately the number to be expected from the other 

 species examined at sizes between about 200 and 220 mm. 

 The total number of plates in the series carmot be counted 

 externally because only about a third to a half or fewer 

 have any of their surfaces exposed, and the total number 

 can only be seen by carefully sawing through the jaw just 



to one side of the midline, a laborious procedure. The 

 trituration surface is somewhat oblique to the broad plane 

 of the individual plates, and the exposed surface is 

 covered by several overlapping plates. 



In most diodontids there are a large pair of apertures 

 posteromedially on the dorsal surface of the premaxil- 

 laries just to either side of the midline which open into 

 the large posterior edge of the pulp cavity, but in Chilo- 

 mycterus affinis and orbicularis the apertures on the dor- 

 sal surface of the premaxillaries are smaller, more 

 numerous, and less precisely arranged than in the other 

 diodontids. This character may be particularly prone to 

 change with increasing specimen size. 



The numbers of vertebrae in the diodontids examined 

 show no differences useful in recognizing natural groups. 

 In Chilomycterus the total number varies modally from 

 18 to 23, with 10 to 13 abdominal and 8 to 10 caudal, 

 while in Diodon there are 19 to 21 total, with 10 to 12 ab- 

 dominal and usually 9 caudal. In the single specimens of 



364 



