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 vertical
succession of the strata, like the sudden
changes that occur in fossil Reptiles and Mammalia.*
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 4
* M. Agassiz observes that fossil Fishes in the same formation
present greater variations 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.
f The nodules of clay stone on the coast of Greenland, containing
fishes of a species now living in the adjacent seas,
(Mallotus Villosus) are probably modern concretions.
j Thus the slate of Engi, in the canton of Glaris, in Swit-
Sauroid Fishes in the Order Ganoid.
The voracious family of Sauroid, or Lizardlike
Fishes, first claims our attention, and is
highly important in the physiological consideration
of the history of Fishes, as it combines in
the structure both of the bones, and some of the
soft parts, characters which are common to the
class of reptiles. M. Agassiz has already ascerzerland,
has long been one of the most celebrated, and least
understood localities of fossil Fishes in Europe, and the mineral
character of this slate had till lately caused it to be referred to the
early period of the Transition series. M. Agassiz has found that
among its numerous fishes, there is not one belonging to a
single genus, that occurs in any formation older than the Cretaceous
series; but that many of them agree with fossil species
found in Bohemia, in the lower Cretaceous formation, or Planer
kalk ; hence he infers that the Glaris slate is an altered condition
of an argillaceous deposit, subordinate to the great Cretaceous
formations of other parts of Europe, probably of the Gault.
Another example of the value of Ichthyology, in illustration
of Geology, occurs in the fact, that as the fossil Fishes of the
Wealden estuary formation are referrible to genera that characterize
the strata of the Oolitic series, the Wealden deposits are
hereby connected with the Oolitic period that preceded their commencement,
and are separated from the Cretaceous formations
that followed their termination. A change in the condition of
the higher orders of the inhabitants of the waters seems to have
accompanied the changes that occurred in the genera and species
of inferior animals at the commencement of the Cretaceous formations.
A third example occurs, in the fact that M. Agassiz has, by
resemblances in the character of their fossil Fishes, identified the
hitherto unknown periods of the freshwater deposits of Oeningen,
and of Aix in Provence, with that of the Molasse of Switzerland.
G. T