63 



Recherclies sm- les Poissons Fossiles.^ Par Louis Agassiz, 

 Professor d' Histoire Naturalles a NeuchateL 



To appreciate the study of fishes in general, and of 

 fossils in particular, it is necessary to consider the position 

 of this class in the animal series. Placed superior to 

 the Radiated animals, (Radiata), and the Molluscs, they pre- 

 sent more complete organization, and are subject to greater 

 peculiarities of structure ; we also find their remains to pre- 

 sent more exact geological limits, than those of the inferior 

 animals. We do not find the class of fishes present the same 

 genera, nor even the same families, throughout all the series 

 of geological formations, with the species often differing 

 but slightly in their appearance, as we do in the class of the 

 Zoophytes ; on the contrary, from one formation to another, 

 this class is represented successively by genera very diflfer- 

 ent, referable to the families which also disappear suddenly, 

 as if the complication of a superior organization were un- 

 fitted for long perpetuation without essential modifications of 

 character ; or rather, as if animal life tended more rapidly 

 to diversity in the superior orders of the animal kingdom, 

 than in the lower gradations of nature. On this account it is 

 that fishes, like mammalia and reptiles, have the species but 

 little extended ; in general they are confined in the series of 

 strata, to short vertical distances, even in the different ge- 

 nera, without passing insensibly from one formation into ano- 

 ther, as we find to be the case with certain shells. One of the 

 facts the most interesting that I have observed, is, that I am 

 not acquainted with a single species of fossil fish which is 

 found successively in two formations, although I am acquaint- 

 ed with a great number, which have an extensive horizontal 

 distribution. Yet the fossil fishes are more advantageous 

 to Geology, than any other fossils, from the circumstance of 



* Continued from vol. iii. p, 344, 



