62 EXPLANATIONS. 



truly be regarded as marks of grade in their own 

 class. When we look to the great fundamental 

 characters, particularly to the framework for the 

 attachment of the muscles, what do we find ? — 

 why, that of these placoids — " the highest types of 

 their class !" — it is barely possible to establish their 

 being vertebrata at all, the back-bone having gene- 

 rally been too slight for preservation, although the 

 vertebral columns of later fossil-fishes are as entire 

 as those of any other animals. In many of them, 

 traces can be observed of the muscles having been 

 attached to the external plates, strikingly indicat- 

 ing their low grade as vertebrate animals. The 

 Edinburgh reviewer's "highest types of their class" 

 are, in reality, a separate . series of that class, — 

 generally inferior, taking the leading features of 

 organization of structure as a criterion, — but, when 

 details of organization are regarded, stretching 

 further both downward and upward than the other 

 series ; so that, looking at one extremity, we are 

 as much entitled to call them the lowest, as the 

 reviewer, looking at another extremity, is to call 

 them the highest of their class. Of the general 

 inferiority, there can be no room for doubt. Their 

 cartilaginous structure is, in the first place, 

 analogous to the embryotic state of vertebrated 



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