CONDITIONS AFFECTING THE DISTRIBUTION OF FOSSILS. 
65 
between the lithological and zoological contents of rocks of the same age in the 
most distant countries. 
Again, in passing from the Baltic governments into the central country of Orel 
and Yoroneje, we perceive, that with the change in lithological features which has 
been described (viz. red sands, marls and calcareous flags prevailing in the for- 
mer, and yellow magnesian limestones and marls in the latter), a corresponding 
change occurs in the relative proportion of mollusks and fishes. In the one, the 
ichthyolites predominate in number of genera and in the profusion of species ; and 
in the other, whole bands are loaded with characteristic mollusks, the remains of 
fishes being more rarely found among them. 
These facts concerning the relative distribution of different classes of the sub- 
marine fauna of an ancient period, the earliest in this part of the world in which 
Vertebrata have yet been discovered, present strong analogies to the manner in 
which the adjacent parts of the present seas are inhabited. The great receptacles 
of fishes are often, if we mistake not, deep sandy bottoms, in which comparatively 
few shelly creatures exist, whilst the latter are prone to congregate in multitudes 
towards the shores, where calcareous springs and other favouring circumstances 
attract them. 
Abandoning, for the present, these general zoological analogies, we might now 
proceed to cite the language of Professor Agassiz and Professor Owen, who have 
each contributed to this work, by developing the character of the ichthyolites of 
the Devonian system of Russia ; but we refer to their own words, as given in the 
Third Part of the volume. We must, however, anticipate their remarks so far as 
they bear upon our general conclusions. We have already alluded to such of those 
fossil fishes as were discovered by M. Pander and Professor Asmus ; and we know 
that these authors were about to publish works descriptive of the Russian ichthy- 
olites of their respective neighbourhoods (Riga and Dorpat). Not doubting the 
ability with which they will describe such remains, we had too many obvious and 
pressing reasons to refer all our specimens to Professor Agassiz. Our great object 
is distinctly to place in parallel, the palaeozoic types of Russia with those of the 
countries with which w x e are acquainted, through our own labouis in the field, and 
the fossils of which have been described by good naturalists. Among the latter, 
it is well known that Professor Agassiz has, from the commencement of our palae- 
ozoic researches, taken the lead in describing ichthyolites. With his pow x eiful 
mind, and by having at his disposal the remains of fossil fishes from many 
