320 L, Agassiz on Animals in Geological Times. 
neously with those of the inferior oolite, or these with those of 
the Oxford clay, or these with those of the upper division of the 
so-called oolitic formation. 'The same may be said of the differ- 
ent natural subdivisions of the cretaceous formation, and of the 
subdivisions introduced of -late among the paleozoic rocks, by 
Sir Roderick Murchison, and Professor Sedgwick, and in America, 
by Professor J. Hall. ; 
But even after this separation of the fossils, the synchronism of 
which may be fully established, our task is only fairly laid open, 
for then must begin the zoological identification of all the spe- 
cies, which has to be correct in every respect before general con- 
clusions can be drawn from it. 
n the first place the specific identity of organic remains is not 
so easily ascertained as many geologists would seem to suppose, 
if we judge from their statements; but unless the validity of @ 
species is sanctioned by a practiced Zodlogist, it can not be taken 
as a basis for sound generalizations in reference to questions of @ 
purely zoological character. The number of false identifications 
which have been accumulated in geological works is truly fright- 
ful. It would be however very unjust to accuse geologists in 
general of inaccuracy for this, the fault is mostly to be traced to 
other parties from which the names were obtained. It should 
es 1S 
three-fold: 1, different species may be considered as identical, 2, 
specimens of the same species in different states of preservatiols 
or of different age, or sex, é&c., may be considered as distinct 7. 
cies, or 3, the same species may have been described by di io 
authors under different names, and their identity afterwards over 
