274 L. Agassiz on the Primitive Diversity 
shores of our temperate regions, and that the temperature of 
the tertiary period having been warmer, we may expect a 
larger number of fossil species from those deposits, I would 
only refer to local enumerations of marine shells from several 
tropical regions, to sustain my assertion that the number of 
fossil shells of the eocene beds of the immediate vicinity of 
Paris is much greater than that of any local fauna of the 
present period, even within the tropics. A catalogue of not 
quite three hundred species of shells, given by Dufo, as occur- 
ring around the Sechelles Islands, the extent of which may 
fairly be compared with that of the lower tertiary beds around 
Paris, will suffice to show, that in a tropical local fauna the 
number of species known to exist in the present day is far 
inferior to the number of species known to have occurred 
during the deposition of the lower tertiary beds in the 
vicinity of Paris. Another catalogue by Sganzin, of the 
shells found about Mauritius, Bourbon, and Madagascar, 
gives also less than three hundred species for that extensive 
range of seas surrounding those islands. Let us further 
compare the results of the investigations of the shells of the 
Red Sea by Hemprich, Ehrenberg, and Riippel, and there 
again we find a smaller number, and a more limited variety 
of types than are found in the tertiary of Paris ; for the whole 
basin of the Red Sea has thus far yielded only 400 species 
of shells. Let us finally take the most accurate survey of 
this kind we have of any shore—that of Panama by Professor 
Adams, extending over 50° of latitude, 28° N. of the equator, 
and 22° S. of it, including the most favourable localities for 
the growth of shells in the Pacific under the tropics, and yet 
we shall find this list exceeding but little the number of 500 
species. In this instance, again, we find that the advantage 
in number and variety is in favour of the tertiary period, and 
not of the present age. If a different result has been ob- 
tained 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 
globe. But let us trace these comparisons through other 
geological periods, with reference to other classes also, and 
