156 CLASSIFICATION OF FISHES, 



not militate against our other analogies. So far^ how- 

 ever_, is this from being true in the present instance, 

 that we actually find the SturionidcB coming opposite 

 to the Amphibia, when we bring the circles of the 

 Vertehrata and the Cartilagines together : there are 

 even some considerations which strengthen the analogy 

 thus inferred. The sturgeons, hke the Amphibia, have 

 no true teeth, and they live in two distinct modifications 

 of the same element, — that is, both in salt and fresh 

 water. The simple fact, however, of their standing in 

 the order of affinity (for this is the primary consi- 

 deration) between the ChimcEridcE and the Polyodonidce, 

 and that the Amphibia hold the same rank between the 

 reptiles and the fishes, is a sufficient argument that they 

 represent each other, although we are not prepared to 

 state the true manner in which this law of nature is 

 effected. We now come to the Polyodonidce and the class 

 Pisces. We are to inquire under what view we may 

 consider the former as a representation of the latter : it 

 is not sufficient to say that Polyodon is a fish, because 

 so are all the Cartilagines. Now, if the question was 

 asked, "WTiat are the most prominent characteristics of 

 the typical orders, independent of their general form ? 

 the answer would be, that such fish possessed free 

 laminated gills, with a large and unconfined branchial 

 opening. These, then, are the very characteristics of 

 Polyodon ; and as they are found in no other type of the 

 cartilaginous circle, it follows that this division, more 

 than any other we have noticed, gives us the best repre- 

 sentation of the ordinary and typical structure of the 

 class Pisces. 



(137.) We have had frequent occasion to remark, 

 ■while tracing the analogies among quadrupeds and 

 birds, that, to illustrate all the peculiarities of an animal, 

 one table of comparisons is not sufficient : many others 

 would then remain ; and we can only explain these by 

 instituting other comparisons, and applying further tests 

 to the accuracy of our theories. Now, the aberrant 

 groups of the order before us particularly require his. 



