CHANGES OF GENERA, AND SPECIES. 271 
nearest to existing genera and species, in the 
most recent Tertiary deposits; and differ from 
them most widely in strata whose antiquity is 
the highest; and that strata of intermediate age 
are marked by intermediate changes of ichthyo- 
logical condition. 
It appears still further, 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 animals, 
and in fossil vegetables; and also in the mineral 
condition of the strata.* 
It is satisfactory to find that these conclu- 
sions are in perfect accordance with those to 
which geologists had arrived from other data. 
The details that lead to them, will be described 
by M. Agassiz, in a work of many volumes, and 
will form a continuation of the Ossemens Fos- 
siles of Cuvier. From the parts of this work 
already published, and from communications by 
the author, I select a few examples, illustrating 
* The genera of Fishes which prevail in strata of the Carboni- 
ferous order are found no more after the deposition of the 
Zechstein, or Magnesian limestone. Those of the Oolitic series 
were introduced after the Zechstein, and ceased suddenly at the 
commencement of the Cretaceous formations. The genera of the 
Cretaceous formations are the first that approximate to existing 
genera. Those of the lower Tertiary deposits of London, Paris, 
and Monte Bolca, are still more nearly allied to existing forms; 
and the fossil Fishes of Oeningen and Aix approximate again yet 
closer to living genera, although every one of their species appears 
to be extinct. 
