272 CHANGES OF GENERA SUDDEN. 
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 same genera, or even 
the same families, pervade successive series of 
great formations; but their changes take place 
abruptly, at certain definite points in the ver- 
tical succession of the strata, like the sudden 
changes that occur in fossil Reptiles and Mam- 
malia.* Not a single species of fossil Fishes 
has yet been found that is common to any two 
great geological formations; or living in our 
present seas.f 
One important geological result has already 
attended the researches of M. Agassiz, viz. 
that the age and place of several formations 
hitherto unexplained by any other character, 
have been made clear by a knowledge of the 
fossil Fishes which they contain.;}; 
* M. Agassiz observes that fossil Fishes in the same formation 
present greater variations of species at distant locahties, 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. 
t The nodules of clay stone on the coast of Greenland, con- 
taining fishes of a species now living in the adjacent seas, 
(Mallotus Villosus) are probably modern concretions. 
X Thus the slate of Engi, in the canton of Glaris, in Swit- 
