The Life and Writings of Agassiz. 9 



Cuvier makes two general divisions among Fishes, the Os- 

 seous and the Cartilaginous Fishes. Agassiz also separates 

 the Osseous fishes from the Cartilaginous, of which he makes 

 his first order that of the Placoidians ; but he divides the 

 Osseous fishes again into three other equally important or- 

 ders ; so that the class of Fishes is divided into four orders ; 

 namely, 1. The Placoidians ; 2. The Ganoidians ; 3. The 

 Ctenoidians ; and, 4. The Cycloidians. This classification is 

 not founded on the skeleton, like that of Cuvier, but on the 

 nature of the outward integuments, the scales. Agassiz 

 starts with the principle, that the outward covering of fishes 

 is the reflex of their internal organisation. With this prin- 

 ciple, he examines the different families of the class of fishes, 

 with respect to their scales, and finds in the conformation of 

 the external integument a variety of characters on which he 

 founds his classification. As to this, it is to be remarked at 

 the outset, that all the osseous fishes, with the exception of 

 a few genera, are furnished with horny scales ; while the 

 skin of the cartilaginous fishes is covered with plates or spines 

 of a peculiar form, known under the names of shagreen, &c. 

 The scales of the Osseous fishes are constructed on a totally 

 different plan, and the differences are so marked, that M. 

 Agassiz considered them a sufficient foundation for his three 

 orders of Cycloidians, Ctenoidians, and Ganoidians. The two 

 former, which comprise almost all the Osseous fishes of the 

 present epoch, both have horny scales ; but they differ in this, 

 that the Ctenoidians have the posterior edge of the scales in- 

 dented, while in the Cycloidians this border is entire. He 

 seeks to prove, that this distinction, apparently insignificant, 

 is, in truth, founded in nature, being the expression of a fun- 

 damental character which reveals itself equally in other parts 

 of the body. Thus, fishes having indented or pectinated 

 scales, have generally prickles on the head, the opercula, and 

 various parts of their body ; while the others, the Cycloi- 

 dians, are smooth and without defence. M. Agassiz considers 

 the perch, with the analogous species, as the type of his order 

 of Ctenoidians ; and the family of the carp, salmon, pike, &c. 

 as typical of the Cycloidians. This division corresponds, 



