176 Prof. Agassiz on a New Classification of Fishes, 



coidians and Ganoidians, which are now far from numerous, but 

 which existed during the whole period which elapsed since the 

 earth began to be inhabited, to the time when the animals of the 

 greensand lived. These remarkable conclusions, to which M. 

 Agassiz had come from the study of more than 600 fossils on 

 the Continent, have been corroborated by the inspection of more 

 than 250 new species, found in British collections. 



The author next observes, that, in the class of fish, more con- 

 siderable differences may be remarked within narrow geological 

 limits, than among inferior animals. We do not see in the class 

 of fishes the same genera, not even the same families, pervading 

 the whole series of formations, as takes place among zoophytes 

 and testacea. On the contrary, from one formation to another, 

 this class is represented by very different genera, referable to fa- 

 milies which soon become extinct, asif the complicated structure 

 of a superior organization could not be long perpetuated with- 

 out important modifications ; or rather, as if animal life tended 

 to a more rapid diversification in the superior orders of the ani- 

 mal kingdom, during equal periods of time, than in its lower 

 grades. With respect to this, it is with fish nearly as with 

 mammifers and reptiles, whose species, for the most part but 

 little extended, belong, at a short distance in the vertical series, 

 to different strata, without passing insen*sibly from one formation 

 to another, as is generally admitted to be the case with certain 

 shells. One of the most interesting facts which Mr Agassiz has 

 observed is, that he does not know a single species of fossil 

 fish, which is found successively in two formations; whilst he is 

 acquainted with a great number which have a very considerable 

 horizontal extent. But the class of fish presents besides to zoo- 

 logical geology, the immense vidvantage of traversing all forma- 

 tions. Thus they afford us the only example of a great division 

 of vertebrated animals, in which we may follow all the changes 

 experienced in their organization, during the greatest lapse of 

 time of which we possess any relative measure. 



The fish of the tertiary formations approach nearest to recent 

 fish, yet hitherto the author has not found a single species which 

 he considers perfectly identical with those of our seas, except 

 the little fish which is found in Greenland in geodes of clay, and 

 whose geological age is unknown to him. 



