12-i HIGH STANDING OF THE PLACOIDS. 



they excel other fish ; but such features are partly partaken 

 of "by families in inferior sub-kingdoms, showing that they 

 cannot truly be regarded as marks of grade in their own class. 

 When we look to the great fundamental characters, particu- 

 larly 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 generally 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 indicating 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 farther, 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 Keviewer, 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 embryonic state of verte- 

 bra ted animals in general. The maxillary and intermaxillary 

 bones are in them rudimental. Their tails are finned on the 

 under side only, — an admitted feature of the salmon in an em- 

 bryonic stage ; and tbe mouth is placed on the under side of 

 the head, — also a mean and embryonic feature of structure. 

 These characters are essential and important, whatever the 

 Edinburgh Reviewer may say to the contrary : they are the 

 characters which, above all, I am chiefly concerned in look- 

 ing to, for they are features of embryonic progress, and em- 

 bryonic progress is the grand key to the theory of develop- 

 men:;." 



