edges are straight and the other edges rounded, in all spe- 

 cies examined except for C. orbicularis. In C. orbicularis 

 the trituration plate is smaller than in the other species, 

 the division into right and left halves is less distinct, and 

 there are two or more series of far smaller and less wide 

 teeth in each half of the plate. In fact, the individual 

 teeth in the trituration plate of C. orbicularis are not 

 much larger than those of the biting edge, at least in 

 adults, in which the number of series of teeth increases 

 greatly over the two or three to each side found in young 

 specimens, while at the same time the width of the rows, 

 especially of the innermost row, is greatly decreased. 



Having numerous series of trituration teeth to each 

 side of the midline and not greatly different in size from 

 those in the biting edge would seem to be a more 

 generalized condition than that of a single series of large 

 wide plates to each side of the midline and greatly dif- 

 ferent from the teeth of the biting edge. Such numerous 

 series of small trituration teeth are also found in the 

 triodontids, the most generalized gymnodonts. However, 

 C. orbicularis is highly specialized in certain other 

 features of the skull, and tetraodontids, the closest 

 relatives of the diodontids, are more generalized than 

 diodontids and usually have only a single series of small 

 trituration teeth to each side of the midline, when they 

 occur at all. Moreover, the trituration teeth in the 

 earliest known diodontids, in the Eocene, are large 

 transversely elongate units that occur in a single series to 

 each side of the midline, just as in all Recent species ex- 

 cept C. orbicularis. I suspect that the numerous tritura- 

 tion teeth of C. orbicularis are a de novo acquisition of 

 this single specialized diodontid and thus not the reten- 

 tion of an ancestral condition such as found in triodontids. 

 The numerous small trituration teeth of C. orbicularis 

 can be considered just as specialized in their own way as 

 the far larger trituration teeth in a single series to either 

 side of the midline found in all other diodontids. 



The jaws of C. orbicularis tend to be less massive and 

 to have a sharper biting edge than in other diodontids, 

 and this, in conjunction with the differences in the tri- 

 turation plates, probably indicates that C. orbicularis 

 has a diet of less hard bodied food than do the other dio- 

 dontids. 



In the large trituration plates as found in the great 

 majority of diodontids, the wide individual tooth plates 

 are formed at the base of the pulp cavity and have a 

 slightly concave smooth upper surface and a slightly con- 

 vex papillate lower surface. The individual plates mi- 

 grate toward the exposed surface of the trituration plate 

 to replace those worn away through use as new plates are 

 continuously being formed at the edge of the pulp cavity. 

 As they move toward the exposed trituration surface the 

 individual plates become increasingly flattened and 

 more closely cemented together. The number of individ- 

 ual plates in the single series to each side of the midline 

 usually increases greatly with increasing specimen size, 



Figure 296.— Diodon hystrix: cross section 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, 88.3 mm SL, B, 228 mm SL, and C, 505 mm S 



