332 M. Agassiz on Fossil Fishes, 



size. Such a number of species added to the inventory of the 

 animal kingdom is an important acquisition, a real advance- 

 ment in zoology, particularly in a class so little known as that 

 of fishes. 



So many new species could not enter into the systems of 

 Ichthyology, without causing a necessity for new changes, both 

 by discovering types entirely new, and making us better ac- 

 quainted with the affinities of various groups and families. 

 M. Agassiz, accordingly, has not confined himself to the 

 establishment of a number of species, genera, and even fami- 

 lies. He has created a classification entirely new, founded, in 

 a great measure, on the importance of fossil fishes. Cuvier 

 admits two great divisions in the class of fishes, the osseous 

 fishes, and the cartilaginous fishes. M. Agassiz likewise sepa- 

 rates the osseous from the cartilaginous fishes, of which he 

 composes his first order, that of Placoid ; but he further 

 divides the osseous fishes into three other orders of the same 

 value, so that the class of fishes is thus divided into four orders, 

 which are, l^^, The Order of the Placolds ; 2d, The order of the 

 Ganoids ; Sd, Order of the Ctendids ; Ath, Order of the Cycloids. 

 This classification is not founded on the skeleton, like that of 

 Cuvier, but on the nature of the exterior integuments, the 

 scales. M. Agassiz lays it down as a principle, that the ex- 

 terior integuments of fishes are the reflection of their internal 

 organization. Proceeding on this, he examines the difi^erent 

 famihes of the class of fishes in relation to their scales ; and he 

 finds in the conformation of the exterior cuirass a multitude 

 of characters, on which he founds his classification. In this 

 point of view, it is, first, to be observed, that all the osseous 

 fishes, with the exception of certain genera, are covered with 

 corneous scales, while the skin of the cartilaginous fishes is 

 furnished with plates or spines of a particular form, known 

 by the name of shagreen in Sharks, and houcles among Rays. 

 The scales of the osseous fishes are constructed on an entirely 

 different plan : the differences are even so decided, that they 

 have appeared sufficient to M. Agassiz to serve as a basis 

 to the three orders Cycloid, Ctenoid, and Ganoid. The 

 Cycloids and the Ctenoids, which comprehend nearly all the 

 osseous fishes of our era, are both possessed of corneous scales ; 



