1 843-44.] POISSONS FOSS/LES. 213 



during which the forms of the oceans and the conti- 

 nents have been successively modified, and each one 

 of them has been characterized by a special flora and 

 fauna ; that is to say, by an ensemble of vegetables and 

 animals specifically different from those coming before 

 or after. The fishes have existed since the oldest ages 

 of the globe, and their remains are found in all the 

 successive periods. Their palaeontological history, con- 

 sequently, is most important, and furnishes precious 

 data concerning this succession of faunas. 



" When Agassiz began his researches and foresaw 

 the importance of the result that he might draw from 

 them, the classification of fishes was not advanced 

 enough to allow sufficient comparisons. Some dissimi- 

 lar forms were associated together, while other very 

 similar ones were separated by large intervals. Before 

 everything else, it was necessary to establish an exact 

 classification. Agassiz found in the scales the neces- 

 sary elements to solve the problem, and he recognized 

 that these teguments of the body correspond well with 

 the interior characters, and that their variations are, 

 in general, associated with and due to, organic differ- 

 ences. He accordingly divided fishes into the follow- 

 ing four orders : (i) The Cycloids, with scales, rounded, 

 smooth, and simple at the margin, composed of laminae 

 of horn or bone, but without enamel, - - endo-skeleton 

 ossified ; (2) the Ctenoids, with scales jagged or pecti- 

 nated (like the teeth of a comb) on the posterior margin, 

 formed by laminae of horn, but without enamel,- -endo- 

 skeleton ossified; (3) the Ganoids, with angular scales 

 regularly arranged like paving-stones, and composed of 



