CHANGES OF GENERA, AND SPECIES. 207 



It appears still farther, that all the great changes in the 

 character of fossil Fishes take place simultaneously with the 

 most important alterations in the other classes of fossil ani- 

 mals, and in fossil vegetables; and also in the mineral con- 

 dition of the strata.* 



It is satisfactory to find that these conclusions are in per- 

 fect accordance with those to which geologists had arrived 

 from other data. The details that lead to them, will be de- 

 scribed by M. Agassiz, in a work of many volumes, and will 

 form a continuation of the Ossemens Fossiles of Cuvier. 

 From the parts of this work already pubhshed, and from 

 communications by the author, I select a few examples 

 illustrating the character of some of the most remarkable 

 families of fossil Fishes. 



It appears that the character of fossil Fishes does not 

 change insensibly from one formation to another, as in the 

 case of many Zoophytes and Testacea ; nor do the sarpe 

 genera, or even the same families, pervade successive series 

 of great formations ; but their changes take place ahrwpily, 

 at certain definite points in the vertical succession of the 

 strata, like the sudden changes that occur in fossil Reptiles 

 and Mammalia.f Not a single species of fossil Fishes has 



* The genera ofFislies wliich prevail in strata of the Carboniferous order 

 are found no more after tlic deposition of tlie Zechstein, or Magr*sian lime- 

 stone. Those of the Oolitic series were introduced after tlie Zcehstein, and 

 ceased suddenly at the commencement of the Cretaceous formations. The 

 genera of the Cretaceous formations are the first that approximate to exist- 

 ing genera. Those of the lower Tertiary deposites of London, Paris, and 

 Monte Bolca, are still more nearly allied to existing forms ; and the fossil 

 Fishes of Oenirgen and Aix approximate again yet closer to living genera, 

 although every one of their species appears to be extinct. 



t M. Agassiz observes that fossil Fishes in the same formation present 

 greater variation of species at distant localities, than we find in the species 

 of shells and Zoophytes, in corresponding parts of the same formation ; and 

 that this circumstance is readily explained by the greater locomotive powers 

 of this higher class of animals. 



