44? CLASSIFICATION OF FISHES. 



as in their prototypes, by the sharpness of the jaw- 

 bones, which are remarkably strong : the analogy, how- 

 ever, here ceases ; for the jaws of the true chelonian 

 reptiles are entire; whereas those of the Balistida are 

 divided in such a manner that they wear the appear- 

 ance of being like the true and distinct teeth, placed in 

 a single row, of ordinary fishes. Finally, we perceive 

 sub-genera, and even species, as they are now classed, 

 scattered among the greatest number of these toothed 

 races, which have the jaws entirely smooth; a clear proof, 

 if any other were wanting, that genera built entirely on 

 these organs are more likely to be artificial than natural; 

 indeed, we have only to look to those among the Si- 

 luridcs, as they stand in the most recent systems, for a 

 justification of this opinion. It frequently happens 

 •that in natural groups, like the last, the teeth offer no 

 variation of the least importance ; while in others they 

 are scarcely the same in two species, and vary in the 

 most remarkable manner, even in the same fish, at 

 different stages of its growth. This is particularly ob- 

 servable among the salmons, and even in the family of 

 sharks. The value of a zoological character is well 

 known to be proportioned according to its prevalence 

 in groups or individuals, which, in every other cha- 

 racter they possess, show a clear and unquestionable 

 affinity. Thus the prevalence of the spiny or of soft 

 rays in the osseous fishes indicate, with other peculiar- 

 ities, the two great typical divisions ; and thus, from 

 its prevalence among families and genera, clearly re- 

 lated, we infer its primary value. But when, in 

 another group, we observe the teeth vary in almost 

 every third or fourth species, although their charac- 

 ters in other respects are precisely the same, it is 

 quite clear that we must look for some other marks 

 of discrimination, possessed by all these individuals, 

 whereby to preserve in our systems that bond of union 

 which we see in nature. Among the sturgeons, for 

 instance, we have some species with teeth, and others 

 without ; vet there is no other difference. Still more 



