L. Agassiz on Animals in Geological Times. 311 
over 50° of latitude, 28° N. of the equator, and 22° S. of it, in- 
cluding the most favorable localities for the growth of shells in 
the Pacific under the tropics, and yet we shall find his list exceed- 
ing but little the number of 500 species. In this instance again 
we find that the advantage in number and variety is in favor of 
the tertiary period, and not of the present age. If a different 
result has been obtained by the estimates made before this, it is 
OWing to the circumstance, that the fossils known from a few 
localities within narrow geographical limits were compared with 
the living species known to occur upon the whole surface of the 
Gove. But let us trace these comparisons through other geolog- 
leal periods, with reference to other classes also, and we shall find 
mM every instance, similar results. The tertiary fossils of Bor- 
deaux, though less numerous in species than those of the eocene 
In the vicinity of Paris, will compare with any local fauna of the 
present period as favorably for variety and number of species as 
those of the lower tertiaries. This may be said, with the same 
certainty, of the tertiary shells of the Sub-appennine Hills, or of 
those of the English Crag of which we now possess a very com- 
plete list. 
If from the tertiary periods we pass down to the cretaceous, 
do we not find in the deposits of Meestricht, or in those of the 
age of the white chalk, a number and variety of shells as great 
as that which may be found on any shore or in any circumscribed 
Marine basin of an extent at all comparable with that of the cre- 
faceous beds within similar limits? Do-we not find in the lower 
ctetaceous strata such as the green sand or the Neocomien, other 
assemblages of the remains of Mollusks, which, in number and 
atiety, are not inferior to those of the white chalk? The 
Sdlitic series, again, will stand a similar comparison quite as well. 
We need not even take the whole group of those deposits, but 
Consider each subdivision of the Jurassic period by itself, and 
Sull we find in every one, local faunee of Mollusks, assuming of 
Course, a different character from those of the cretaceous or ter- 
tary, but nevertheless’ sufficiently diversified to admit of an 
*stimate, as advantageous, with respect to the points under 
Consideration, and to the local faunee of the present day as to the 
us assemblages of fossils, or those of the tertiary period. 
Of course, in accordance with the peculiar character of the age, 
diff ent families prevail in these different periods; the Cepha- 
lopoda are extremely numerous and surprisingly diversified during 
the cretaceous and Oolitic periods; while they dwindle down to 
