284 LAKE SUPERIOR. 



is, on the contrary, in our species, provided with such serratures as 

 is the case in P. coenosus. 



Such are the principal features upon which the comparison may 

 rest, while good figures are yet wanting. The differences which we 

 have indicated, however slight they be, do not allow us to identify 

 our species with the one or the other of those mentioned above. 

 The comparison of original specimens would be necessary in order 

 to fix in a sure manner the traits of resemblance, or the differential 

 characters of each of them. 



PERCOPSIS, Agass. 



In order fully to understand and perfectly to appreciate the char- 

 acters of this genus, and the interest involved in its discovery, it is 

 necessary to remember various relations of the different types of the 

 whole class, which however do not constitute generic distinctions, 

 although they bear upon the peculiarities of this new type. 



In the first place, it is a matter of no little importance that, 

 ' among the fishes of former ages, we find every where types which dif- 

 fer widely from the forms of our time, and that those forms are the 

 more different, as they belong to older geological deposits. The 

 differences are even so great, that out of the four orders of this class, 

 there are only two which constitute the fauna of fishes in the older 

 formations ; two orders, which in our day are comparatively re- 

 duced, I mean the Placoids and Ganoids. Moreover, the types are 

 peculiar in all epochs. For instance, the sharks of former days, espe- 

 cially those of older epochs, resemble solely that curious genus of 

 Port Jackson, New Holland, the Cestracion, which is so remarkable 

 among the living fishes as to form a group by itself. The Ganoids, of 

 which there are so remarkably few in the present creation, such as 

 the gar-pike (Lepidosteus) of this continent, are not less peculiar, 

 and in connection with those ancient Placoids, constitute the only rep- 

 resentatives of the class of fishes throughout the earlier geological 

 ages down to the deposits of the chalk, when new families of other 

 orders, the Ctenoids and Cycloids, begin to make their appearance, 

 preparatory as it were to the present development of that class, and 

 are successively diversified with the modified adaptations of the whole 



