THE NATIONAL INSTITUTION. 
169 
The singular tribes of animals studied by the conchologist, are known to be ex- 
tensively spread over every part of the globe visited by man, and that they likewise 
affect some situations in greater numbers, size, and variety of species, than others. 
In those countries whose zoology has been more fully explored than our own, as 
France, Germany, and England, it has been ascertained that soil and climate are 
the two great causes which modify their distribution. In reference to the first 
cause, limestone rocks and the superincumbent soil seem to be more favorable than 
any others, to the number and propagation of species. The next more favorable 
are the sandstones; basalt, granite, argillaceous, and slaty rocks, generally exer¬ 
cising a negative or unfavorable influence. The latter cause, climate, produces an 
unquestionable diminution in the number, size, and variety of species, bearing a 
general proportion to the lowness of the temperature, since, as we advance north¬ 
ward, many species disappear altogether. In temperate situations, individual 
species multiply to greater extent, and in warm districts they display a greater 
superiority in respect to color. Sheltered vallies, places of low elevation, with a 
southern exposure and a proximity to the sea-coast, afford the greatest advantages 
for the propagation of these tribes. 
Guided by these or similar views, the surface of the United States may be di¬ 
vided into a number of regions or districts, for the purpose of collecting its shells, 
and from each, as the soil or climate varies, we must expect to receive suites of 
specimens differing widely from each other. That we may assign some general 
limits to these portions of country, it will be necessary, first, to sketch out a few 
grand divisions, to be subdivided afterwards. If we travel, for instance, from the 
Atlantic ocean towards the great lakes, there will successively occur several marked 
lines or belts of country where the species must be changed to suit the soil, climate, 
elevation, temperature, and the salt or fresh condition of the water. In the sea, 
there will be found, and there may be dredged from the sub-merged coast, such as 
are peculiar to the sea, and may be designated as marine species. At the mouths 
of rivers and estuaries, and in the brackish water, which extends as far inland as 
the tide flows, another suite of species, peculiar to this combination of circum¬ 
stances, will be detected. Beyond the point of high tide the water remains fresh, 
and here are found those which are known and designated as fluviatile species. Of 
the terrestrial species, a diversity equally striking may be noticed, by returning to 
the edge of the ocean, collecting all the species inhabiting the low flat lands, com] 
posed of sandy and clayey soil, extending, for the most part, along the whole coast 
of the United States. Compare these with such as inhabit the sides and summits 
of the low ranges of hills or terraces constituting the head of tide, and a striking 
difference will be at once perceived. These again are, in many respects, dissimilar 
from those living on the broad belt of the tertiary formations, which stretch along 
the base of the eastern slope of the primary Atlantic chain of mountains. Upon 
the sides and summits of these, again we discover new genera and species; and we 
notice, with surprise, that those which abound on the eastern slope, in but few 
cases cross over the ridge to the other or western aspect of the same chain. The 
most remarkable difference takes place, however, in the fresh water bivalves, espe¬ 
cially of the genus unio, which becomes suddenly changed, from an elongated 
form and very thin shell, to swarms of new species, of great size, endless variety of 
form, and of astonishing thickness. These characteristics are maintained through- 
